I confess my first reaction on reading this comment was "You mean the way Mastodon emerged a year ago and made Twitter completely irrelevant?"
It's possible that Facebook will be disrupted (although I don't think it's an exaggeration to say that Facebook is operating at such an unprecedented scale that it's hard to apply traditional "rules of thumb" to them), and it's possible that what disrupts them will be a nonprofit competitor--but it's not going to disrupt them because it's nonprofit. Bluntly, idealism isn't sufficient; I don't see much indication that it's even necessary. Wikipedia "won" in its space against early competitors largely because of the features that its competitors perceived as its big weakness, e.g., not only can anyone read it for free, anyone can edit it for free, without even making an account.
A rule of thumb that I do think will prove to hold up: whatever replaces Facebook will be at least as difference from FB as FB was from MySpace. We're seeing that to some degree in the interest shift from FB to Instagram and SnapChat. (Of course, FB owns Instagram; another thing that's going to make them really difficult to disrupt as a corporation rather than a service is that they're big enough to throw money at potential disruptors until they sell out.) I think it's also going to be very difficult for a nonprofit to disrupt them for that reason, though, honestly; remember, even as huge as Wikipedia is, they're constantly struggling for money, and their infrastructure is way less resource-intensive than something like FB's is. Thing That Replaces FB may solve that by being fully decentralized and distributed, but it needs to be just as easy to use as FB is (and again, provide a more compelling reason to use it rather than "it's not FB").