I think the real problem is that Facebook is for profit and, as long as it can get away with it, it's against its interest to let people easily move "their existance" elsewhere. So unless it MUST allow this (maybe some regulation? or serious competition), form the business POV, it makes no sense for them to do it...
P.S. As for the people joining these things, if they *want* to have the kind of "existence" Facebook provides, they don't have much of a choice, do they? Where are they going to go? Whatever other for profit alternatives might be around, they are going to have the same counterincentives.