Yeah, after a certain point, the network effect takes over. That doesn't answer how Facebook got to be big enough for network effect to dominate. Or maybe network effect started at the beginning, because it was school-by-school.
In fact, MySpace is only about 6 months newer, and I think was dominant for a while. It seems like maybe Facebook grew from people becoming dissatisfied with MySpace. I don't think we have seen a similar service growing considerably from dissatisfied Facebook users.
We used to have social centralization, the difference now is that there are a lot more choices. Decades ago, it might have been "everyone watching the same channel", a bit before that, "listening to the same station".