People keep mentioning alternatives to FB but I feel pretty locked-in. I would be interested in joining a site that was a superset of Facebook... that is, a site where I could control and access my Facebook data but also interact beyond Facebook.
That said, it would be a very useful thing to have if someone could get it right.
I say this only half in jest. One can't get 400 million + people to move, but niche sites could certainly peel off a substantial number of users over time. And facebook itself started out as a niche social netorking site...