https://phys.org/news/2019-03-facebook-longtime-product-chief-revamps.html

Cox, 36, worked closely with Zuckerberg through the company's ups and downs, having joined up about 20 months after Facebook was hatched in 2004 in Zuckerberg's Harvard dorm room.

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. "My guess is there was some sort of disagreement. He would not be leaving at this challenging time if there wasn't something else going on."