A long-time Zuckerberg confidante, Cox's departure timing suggests he may not have been on board with the new corporate vision.
"Most all my personal highs and lows of the last decade have been tied up in the journey of this company, with Mark, and with so many of you", Cox wrote in a post. He joined 13 years ago and helped create Facebook's news feed feature.
While Cox gave no specific reason for his move, he noted that Facebook chief Mark Zuckerberg has unveiled a new direction away from being the "digital town square" to focus on smaller-scale, private interactions. He also built Facebook's first human resources teams and oversaw the strategy for the apps that Facebook owns, Zuckerberg wrote.
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Cox "is a great guy who is someone who has always tried to do good", said David Kirkpatrick, an author who became well acquainted with Cox and Zuckerberg while writing a book about Facebook. In the future, Zuckerberg predicted, Facebook's users will be posting more in messaging apps than they do on their public News Feeds.
Facebook isn't hiring another executive to replace Cox. Chris Cox was responsible for Facebook's flagship apps - including WhatsApp, Instagram, and Facebook itself. "Instead, the leaders of Facebook (Fidji Simo), Instagram (Adam Mosseri), Messenger (Stan Chudnovsky), and WhatsApp (Will Cathcart) will report directly to me, and our Chief Marketing Officer (Antonio Lucio) will report directly to Sheryl".
Facebook's family of apps strategy has so far been led jointly by Cox and Javier Olivan, vice president of growth.
Their departures come barely a week after Zuckerberg announced sweeping changes to Facebook, in order re-orient the social network around encryption and privacy.
Daniels took the helm of WhatsApp after founders Jan Koum and Brian Acton left the messaging app a year ago, reportedly over disagreements about the increased integration of WhatsApp's operations with Facebook's.