And users will naturally get unified T&Cs between the platforms, so they can take all that data they trusted with one set of conditions, and migrate it safe in the knowledge that the same conditions apply.
You couldn't give me a barge pole long enough to stay away from SpaceBook.
In any case, once everyone has a few dozen "friends", the concept starts to become very boring and diluted. Ooh look, I have a friend on MySpace who has a "friend" on FaceBook who has Paris Hilton as a "friend", so that means I'm only really three hops away from the silly tart, and the gaping, empty maw of my shabby, bereft life is filled with virtual meaning.
I, for one, welcome our new profile transfer engine developing overlords.