At the rate facebook is currently going, in a couple years all data on facebook will be world-readable. At that point it should be easy for anyone to download their facebook to another site. One could then clone facebook, and at an agreed-upon date, everyone could move over to the new site.

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...

I can't see that being easy. Since most of a social network is your relationships with other users, and those users may or may not exist on other sites (or may have duplicate people with the same name, etc), it seems like it would be quite a difficult problem. You'd also have to somehow convince one or more major providers to use it, but I don't see that being easy either.

That said, it would be a very useful thing to have if someone could get it right.