What does it take for someone to switch to another social network? Actually quite a bit. One, I want my friends to be there too. That alone is pretty difficult, and a common complaint with G+.
But I think the real roadblock is probably photos. People love their photos. Can you imagine the work some people would have to do just to move that all to a new network? Some people I know have thousands of photos of themselves, and have posted thousands of photos. That is no small task. It helps keep people tied to Facebook, in my opinion.
Certainly I think Facebook could be usurped, but I don't think it's trivial and it is probably going to require some competitor finding a good way to help someone move their social life (i.e. photos and friends) trivially.
