Chris Cox, the man behind Facebook's decision to blow up the 'like' button

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 recognised 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. 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.

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.

The solution would eventually be named Reactions, which 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 probably the closest thing Internet users have to an editor-in-chief of their digital life. Cox's team manages the News Feed. Invisible formulas govern what stories users see as they scroll, weighing baby pictures against political outrage.

Cox is frequently seen at Zuckerberg's side. 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."

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. In Silicon Valley fashion, Cox prefers to recast past mistakes as healthy experiments and valuable learning experiences.

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 personalised 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.

Facebook Reactions won't get rid of like - it will be an extension. This update may seem trivial. All it's doing is increasing the number of clickable responses. 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.