https://phys.org/news/2019-03-facebook-depart-company.html

It isn't clear what future plans the executives have. Zuckerberg wrote that Cox considered leaving years ago to pursue new projects, but he decided to stay in 2016, around the time Facebook began taking heat for acting as a conduit for misinformation and Russian election interference. Zuckerberg phrased that more delicately, noting that "We both realized we had too much important work to do to improve our products for society."

Cox joined 13 years ago and helped create Facebook's News Feed feature. Less than a year ago, he was put in charge of all of Facebook's apps, including its flagship social media site as well as Instagram, WhatsApp and Messenger.