Sure, but if I choose a competitor I either have to convince everyone I know to switch or be lonely. This isn't to say that this is Facebook's fault (well, I guess you could make the argument that they haven't tried to connect their network up with others, but that's not really their responsibility either), but it still is a huge pain.

(Similar is the problem with AIM; I don't like it, but few people I talk to use anything else.)