The most drastic change to Facebook in years was born a year ago during an off-site at the Four Seasons Silicon Valley, a 10-minute drive from headquarters. Chris Cox, the social network’s chief product officer, led the discussion, asking each of the six executives around the conference room to list the top three projects they were most eager to tackle in 2015. When it was Cox’s turn, he dropped a bomb: They needed to do something about the “like” button.

The like button is the engine of Facebook and its most recognized symbol. A giant version of it adorns the entrance to the company’s campus in Menlo Park, Calif. Facebook’s 1.6 billion users click on it more than 6 billion times a day—more frequently than people conduct searches on Google—which affects billions of advertising dollars each quarter. Brands, publishers, and individuals constantly, and strategically, share the things they think will get the most likes. It’s the driver of social activity. A married couple posts perfectly posed selfies, proving they’re in love; a news organization offers up what’s fun and entertaining, hoping the likes will spread its content. All those likes tell Facebook what’s popular and should be shown most often on the News Feed. But the button is also a blunt, clumsy tool. Someone announces her divorce on the site, and friends grit their teeth and “like” it. There’s a devastating earthquake in Nepal, and invariably a few overeager clickers give it the ol’ thumbs-up.

Changing the button is like Coca-Cola messing with its secret recipe. Cox had tried to battle the like button a few times before, but no idea was good enough to qualify for public testing. “This was a feature that was right in the heart of the way you use Facebook, so it needed to be executed really well in order to not detract and clutter up the experience,” he says. “All of the other attempts had failed.” The obvious alternative, a “dislike” button, had been rejected on the grounds that it would sow too much negativity.

Cox told the Four Seasons gathering that the time was finally right for a change, now that Facebook had successfully transitioned a majority of its business to smartphones. His top deputy, Adam Mosseri, took a deep breath. “Yes, I’m with you,” he said solemnly.

Later that week, Cox brought up the project with his boss and longtime friend. Mark Zuckerberg’s response showed just how much leeway Cox has to take risks with Facebook’s most important service. “He said something like, ‘Yes, do it.’ He was fully supportive,” Cox says. “Good luck,” he remembers Zuckerberg telling him. “That’s a hard one.”

The solution would eventually be named Reactions. It will arrive soon. And it will expand the range of Facebook-compatible human emotions from one to six.

Cox isn’t a founder, doesn’t serve on the boards of other companies, and hasn’t written any best-selling books. He’s not a billionaire, just a centi-millionaire. He joined Facebook in 2005, too late to be depicted in The Social Network, David Fincher’s movie about the company’s early days. While Zuckerberg manages an expanding portfolio of side businesses and projects—Instagram, WhatsApp, the Oculus Rift virtual-reality headset, a planned fleet of 737-size, carbon-fiber, Internet-beaming drones—Cox runs “the big blue app.” That’s Facebook’s term for the social network that we all compulsively check a few dozen times a day. He’s also the keeper of the company’s cultural flame, the guy who gives a rousing welcome speech to new recruits every Monday morning at 9 a.m. It’s a safe bet that all 12,000 Facebook employees know his name.

He’s probably the closest thing Internet users have to an editor-in-chief of their digital life. Cox’s team manages the News Feed, that endless scroll of Facebook updates. Invisible formulas govern what stories users see as they scroll, weighing baby pictures against political outrage. “Chris is the voice for the user,” says Bret Taylor, Facebook’s former chief technology officer. “He’s the guy in the room with Zuckerberg explaining how people might react to a change.”

Cox’s ascension has been gradual and, for the past few years, clearly visible to Facebook watchers. Many first met him during the 2012 initial public offering roadshow, when the company distributed a video of executives talking about its mission. Along with Chairman and Chief Executive Officer Zuckerberg and Chief Operating Officer Sheryl Sandberg, the film included Cox, who gazed earnestly into the camera at close range while employing some seriously overheated rhetoric: “We are now changing within a generation the fabric of how humanity communicates with itself.”

He’s frequently seen at Zuckerberg’s side. Here are Zuckerberg and Cox running a three-legged race for a company game day, with Cox wearing a banana suit; embracing after Facebook started trading on the Nasdaq (Zuckerberg hugged Sandberg first and Cox second); riding a float together during San Francisco’s gay pride parade.

Zuckerberg says Cox is one of his closest friends and “one of the people who makes Facebook a really special place.” He mentions Cox’s IQ and EQ—emotional intelligence—and how “it’s really rare to find people who are very good at both.” He’s also cool in a way that Zuckerberg, in particular, isn’t. Cox, who moonlights as a keyboard player in a reggae band, dresses fashionably, usually leaving a button open on the top of his neatly tailored work shirts. He’s also irksomely handsome and displays the casual cheer of someone who knows it.

Look a little deeper, though, and Cox’s record isn’t quite as tidy. He’s been in charge of some of Facebook’s biggest duds: a nicely designed news-reading app for smartphones called Paper, which no one used, and a major revamp of the News Feed that was scrapped because it didn’t work well on small screens. If you look at the things poised to deliver big growth opportunities at Facebook—Instagram and WhatsApp being the biggest—they’re mostly acquisitions, not reinventions of the big blue app.

In Silicon Valley fashion, Cox prefers to recast past mistakes as healthy experiments and valuable learning experiences. “I think any good company is trying things, is forcing itself to try things, and you need to be able to put things out there and try and learn,” he says. “People only get in trouble if they’re not honest about failure.”

Cox first heard of job opportunities at Facebook while pursuing a master’s degree in computer-human interaction at Stanford. A roommate already worked there and badgered Cox to interview, primarily because there was a $5,000 recruiting bonus. Cox was skeptical. Wasn’t Facebook just a glorified dating site?

The headquarters back then were on University Avenue, Palo Alto’s main drag. When he got there, co-founder Dustin Moskovitz described Facebook as a crowdsourced directory of everyone. He drew circles on a whiteboard, then lines connecting them to represent “friending” on the site. By looking at each other’s profiles, friends could bypass the first awkward five minutes of every conversation—those rote questions like “where are you from?”—and move on to deeper connections. Cox was riveted.

He dropped out of Stanford (naturally) and joined the company when it had about 30 employees. His first job was developing the News Feed, the feature that made Facebook a global addiction. At the time, though, he and Zuckerberg badly misjudged user reaction: People hated it. They felt as if their private interactions were suddenly being exposed. “It wasn’t our best product rollout,” Cox concedes. He learned that people tend to be suspicious of well-capitalized Silicon Valley startups preaching lofty values such as “openness” and “sharing.”

In late 2007, after Facebook hired its 100th employee, Zuckerberg decided he needed to put someone he trusted in charge of personnel. This became Cox’s strangest career move: Zuckerberg asked him to become the company’s first human resources chief. Zuckerberg now says he thought it was “an opportunity to take a different approach than other companies and to bring a technical spirit to defining all these different aspects” of the company’s culture.

Cox scheduled one-on-one meetings with every employee and became a sort of in-house therapist. “He had to endure the slings and arrows of people’s complaints from all over the company,” Yishan Wong, an early employee, wrote on the community website Quora. “And he did so without becoming a cynical, uncaring shell of a man.”

Cox says the HR job gave him a way of looking at things through other people’s eyes. It also led him to ponder Facebook’s mission in the world, which is when he started reading the works of communications theorist Marshall McLuhan. Each wave of media technology, McLuhan wrote, is initially greeted with anger and mistrust.

That was comforting to Cox, because it explained some of the hostility that Facebook was encountering. “We were in this period back then where people really didn’t understand Facebook and didn’t believe it could become anything,” he says. “McLuhan helped tell that story in a broader context.”

Cox returned to engineering in 2008, but he’s still the company’s cultural ambassador. He weaves McLuhan’s lesson into his Monday morning speeches to the new recruits. The talks usually start with a question: “What is Facebook?” He lets the room hang in silence until someone is brave enough to say, “It’s a social network.” Wrong. Facebook is a medium, Cox says, referring to McLuhan’s famous dictum, “The medium is the message.” In other words, how Facebook presents content and the way in which it allows users to read, watch, comment on, and like that content influences how all 1.6 billion members see the world around them.

Cox spends most of his days in the new Frank Gehry-designed Building 20 on the Menlo Park campus. The structure is a huge, 430,000-square-foot rectangle. A grassy park is on the roof, with a hot dog stand on one side and a smoothie shop on the other. Inside the cavernous space, full of rustic art and chalkboard walls, Facebook employees tie silver balloons to their movable standing desks to mark their “Faceversary,” celebrating how long they’ve worked there. Cox had his 10th Faceversary last fall.

On a Wednesday in November, he enters a conference room for the second of five meetings and confesses that he’s breaking the rules: Executives are discouraged from scheduling meetings on Wednesdays, which is supposed to be a day engineers and designers can work without interruption. Nevertheless, Cox and his team need to talk about tailoring the Facebook smartphone app for India. On a screen at the front of the room, there’s a bar chart of Indian users on Android phones, broken down by the estimated speed of the cellular network they use most often—2G, 3G, and so forth.

“Can you just hang on that stat for a sec?” Cox asks, peering at the chart with his elbows on his knees. “4G is a whopping 0.2 percent.”

“It’s just one guy hanging out there,” says a product manager, Chris Struhar.

The team can’t afford to wait for India to speed up its mobile networks—frustrated users will simply stop using Facebook. (Or worse. The company recently faced street protests in the country for its plan to offer Free Basics, a stripped-down, free Internet service that includes Facebook and not much else.) Struhar proposes to use less data in the app, in part by recycling older stories that don’t have to be freshly downloaded. Cox agrees. “My intuition, which we could prove wrong, is people just want more stuff,” he says. He imagines himself as the user, looking for any hit of digital nicotine that will stave off boredom at, say, a bus stop. “That’s definitely what I want. I just want more stories.” Cox then reviews a couple of other ideas, like a spinning icon on photos that will let users know the app is loading, potentially decreasing what the company calls “rage quits.”

Near the end of the meeting, he wonders aloud how to get other Facebook employees to start thinking about the particular challenge of building features that will work on yesterday’s mobile networks, still in use around the world. Someone proposes switching everyone at the company to a 2G connection once a week. Cox loves the idea. “This is our tool for empathy,” he says. “Happy Wednesday, you’re in Delhi!” Two weeks later, the company implements 2G Tuesdays.

“Empathy” is a word Cox throws around a lot, and which his colleagues often use about him. Facebook blundered in the past when it didn’t take the time to talk to and understand its users. In the old days, product teams tested features in New Zealand, which has the advantage of having an isolated, English-speaking population but is hardly an accurate representation of the world. Under Cox, Facebook’s product team is tackling more sensitive subjects, such as designing a way for accounts to become memorials after someone’s death, or helping users navigate the aftermath of a breakup by selectively blocking pictures of the ex. His goal, which he admits Facebook hasn’t reached, is to make the News Feed so personalized that the top 10 stories a user sees are the same they’d pick if they saw every possibility and ranked it themselves. A side effect of making things easier for users: happy advertisers. Under Cox, Facebook found a way to make advertising work on its smartphone app, and came up with video ads that play automatically.

Since Cox was elevated to chief product officer in 2014, his team has consulted with an outside panel of about 1,000 Facebook users who rate every story in their feed and offer feedback. There are also a handful of product test stations scattered around Facebook’s offices that look a little like interrogation rooms—tiny spaces with brightly lit desks. A camera is attached to a test subject’s smartphone to film their actions while Facebook employees watch through a one-way mirror. Sessions can go on for hours. Sometimes they’re live-streamed to a larger audience of employees.

Cox applied this testing regimen to the revamping of the like button. He wasn’t part of the team that originally developed the button from 2007 to 2009, but colleagues have war stories about how hard they had to work to get Zuckerberg on board. According to longtime executive Andrew Bosworth, there were so many questions about the button—should likes be public or private? would they decrease the number of comments on stories?—many thought the feature was doomed. Even its champions had no idea of the impact it would have on the company’s fortunes. It was simply meant to make interactions easier—just click like on someone’s post about their new job, instead of being the 15th person to say congratulations.

Eventually the button became a crucial part of how Facebook’s technology decides what to show users. If you like beauty tips a friend shares from some Kardashian or other, the software calculates that you should also see ads and articles from People magazine and Sephora. “The value it has generated for Facebook is priceless,” says Brian Blau, an analyst at Gartner.

It’s a way of creating a connection, even if it’s superficial. If users click like on a post about the Red Cross’s disaster relief efforts, they feel as if they’ve done something to help. (In January, Sandberg went so far as to suggest that likes could help defeat Islamic State: By promoting the posts of survivors, users could somehow drown out the hate.) Liking someone’s photo is an awkwardness-free way to make contact with someone you haven’t seen in years. Alternatives to like will let Facebook users be a little more thoughtful, or at least seem to be, without having to try very hard.

Facebook researchers started the project by compiling the most frequent responses people had to posts: “haha,” “LOL,” and “omg so funny” all went in the laughter category, for instance. Emojis with eyes that transformed into hearts, GIF animations with hearts beating out of chests, and “luv u” went in the love category. Then they boiled those categories into six common responses, which Facebook calls Reactions: angry, sad, wow, haha, yay, and love.

The team consulted with outside sociologists about the range of human emotion, just to be safe. Cox knows from experience that he doesn’t have all the answers: When the company redesigned the News Feed in 2013, it looked great on the iMacs in Facebook’s headquarters but made the product harder to use everywhere else. “There are a million potholes to trip over,” Cox said.

Facebook Reactions won’t get rid of like—it will be an extension. Within the company, there was some debate on how to add the options without making every post look crowded with things to click. The simpler Facebook is to use, the more people will use it. Zuckerberg had a solution: Just display the usual thumbs-up button under each post, but if someone on her smartphone presses down on it a little longer, the other options will reveal themselves. Cox’s team went with that and added animation to clarify their meaning, making the yellow emojis bounce and change expression. The angry one turns red, looking downward in rage, for example. Once people click their responses, the posts in News Feed show a tally of how many wows, hahas, and loves each generated.

This update may seem trivial. All it’s doing is increasing the number of clickable responses. People already comment on posts with emojis or, in some cases, actual words. But the feature will probably make Facebook even more addictive. And it will certainly give Cox’s team a lot more information to throw into the News Feed algorithm, thereby making the content more relevant to users—and, of course, to advertisers.

In October the team got close enough to a final design that Zuckerberg felt comfortable mentioning the project in a public interview, giving no details except that there wouldn’t be a dislike button. Cox worried it was too soon to talk about the emotions Facebook picked. (Yay was ultimately rejected because “it was not universally understood,” says a Facebook spokesperson.) Cox says he spent the next morning parsing through responses to the announcement, reading what users thought the social network needed and preparing to start over if necessary.

A few weeks later, the team began testing Reactions in Spain and Ireland, then Chile, the Philippines, Portugal, and Colombia. In early January, Cox flew to Tokyo to sell Reactions to Japan. “You can love something, you can be sad about something, you can laugh out loud at something,” he said to a crowd of reporters at Facebook’s offices in the Roppongi district. “We know on phones people don’t like to use keyboards, and we also know that the like button does not always let you say what you want.”

He explained Facebook’s goal: a universal vocabulary that lets people express emotion as they scroll through their feed. In a sense, Reactions is an adaptation of digital culture in Asia, where messaging apps such as Line and WeChat have already established a complex language of emojis and even more elaborate “stickers.”

Cox says Reactions’ biggest test so far was during the November terrorist attacks in Paris. Users in the test countries had options other than like, and they used them. “It just felt different to use Facebook that day,” he says.

Facebook won’t give a specific date for when Reactions will be introduced in the U.S. and around the world, just that it’ll be “in the next few weeks.” Cox says the data he has looks good and that users will take to Reactions, though he takes pains not to sound in any way triumphant. “We roll things out very carefully,” he says. “And that comes from a lot of lessons learned.”

—With Brad Stone and Hiroyuki Nakagawa

(Updates with Sheryl Sandberg’s clarification of her comments about the Islamic State. Corrects the exchange where Facebook started trading in the tenth paragraph, and the spelling of Julie Zhuo's name in photo caption.)