I don't see what would make basic facebook history hard to move around. Facebook of course already offers a "Download a copy of your Facebook data" function. Future-facebook could import it, or the most fundamental/basic parts of it, probably -- the problem is not the data itself, but the connections in it to other users, it's not that valuable without it. I suspect there isn't much interest from users in things-trying-to-be-facebook of importing the data.

I think it's the ability to communicate that's harder to move around, and why there's such facebook lockin. Your social network and ability to communicate with them in a public/group fashion.