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.)
Walls are irrelevant if you have the whole world inside them. Facebook doesn't need to cooperate with other elements because it is a behemoth. There will always be a minority which wants to see more outside access, and Facebook can appease them, but they don't need to go in that direction for growth. The way out for them is reorganizing and adding more features to tie people further to the service.