The difference is that foursquare isn't a monopoly and facebook is. Quiting foursquare if you don't like them is easy (nothing to loose other than a service you didn't like for whatever reason), quiting facebook because you don't like them can be hard because it has a monopoly over your friends.
When it's hard to quit something that you don't like, that thing can easily evolve in ways against you, essentially you've removed the democratic aspect of the thing and you have a dictatorship - facebook.
I think this problem is a little unique to social networking, it feels like it's inevitable that one will prevail above all others because people _want_ to all be on the same platform. If social network is to survive in a healthy way it would seem that a cross network protocol would need to exist to make it easy to switch between them, but that will likely never come from facebook.