He might be scared that his ad business won't be as profitable in a few years. He'll still be rich, but perhaps that's not enough for him. He has all the correct number and knows exactly how many of their users are bots and how much of the ad revenue comes from scams. Maybe he sees a potential risk that we can't.

Could also be a self-image thing. Facebook isn't exactly young and fashionable anymore, it's more akin to a weird mid-90s idea of a close walled Internet in some sense. Instagram users I'd guess is mostly younger women. None of these really align with Zuckerbergs macho persona.