"One of Chris' biggest skills is emotional intelligence. He's just one of those rare people from an engineering background who excels at that," Facebook engineer Mark Slee said in a 2008 interview.
Cox ascribes Facebook's success to Marshall McLuhan's theory that all new technology, from roads to the printing press, was built to fulfill the basic human desire to connect with others. With Zuckerberg, Cox formulated "social design," the idea that people — who they are, what they do, what they care about, who their friends are — not software code, should shape new features and products.
"He has one of the best fundamental understandings of what Facebook is all about and what it is trying to do," Slee said. "He gets the super zoomed out picture."
Facebook has ruffled users by rolling out new designs and features. It has been frequently condemned for how it handles users' personal information, and even had to reach a 20-year privacy settlement with the Federal Trade Commission.
Chris Kelly, Facebook's former chief privacy officer, said Cox reined in efforts to push the envelope on privacy.
"He has made sure that users are not pushed to be more open than they are comfortable being," Kelly said.
Facebook's financial success rides on keeping its users glued to the service even as they shift their attention from personal computers to mobile devices, on which it's tougher for Facebook to show them ads. The pressure will only intensify next week, when Facebook is slated to begin trading as a public company.
But Cox is known inside Facebook for keeping his cool, whether on stage at annual developer conferences or on keyboards in a packed Berkeley nightclub with Baba Ken and the Afro-Groove Connexion, a mix of Nigerian and American musicians.
"When I play music, it's about connecting with people," Cox said. "Music is like a deeper language. A lot of being a good musician is being a good communicator."