Zuckerberg IS facebook. He controls it. Who's to say even if the main FB app and IG app and WhatsApp apps all lost their entire userbases and revenues went to 0.05% of what they are now, that he wouldn't just run the 500 person skeleton of what they currently are and just focus on his latest pet project, whether it be VR or something else. Honestly that's what I'd do if I was a billionaire with a passion for technology and a failing company but with millions in cash reserves.