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."