I think you're on the right track, but it's not just because it's an "advertising-supported" business model. It's the fundamental laws that govern our society: profit. Replace Zuckerberg, Bezos, et al. with anyone else, and the new CEOs' decisions will be bound by the same constraints.
Thought experiment: Cory Doctorow becomes CEO of Facebook with Zuckerberg's entire stock allocation and equivalent voting control. Do you stand by your assertion that nothing changes?
One of two things happens:
1) Cory Doctorow gradually morphs into Zuckerberg
2) Another social network, run more like Facebook is currently, replaces the one that Cory Doctorow is running. The reason Facebook didn't go the way of Friendster, Livejournal, and MySpace, is that he figured out how to play the game (as it currently exists) better than anyone else. Cory Doctorow would be like someone trying to win the Tour de France when everyone else was doping. In this analogy, Zuckerberg is Lance Armstrong, playing the existing game the way you need to play it to win that game.
Reddit isn't perfect, but it is run totally differently from Facebook, including: offering paid subscriptions, having an open API, and not trying to justify widespread surveillance as an ad-targeting tool.
