The principal barrier to many people leaving Facebook is a manifestation of the network effect. It may be the only place they can easily keep up with a lot of friends.
If something were to replace it, you would need to have a free level to allow the network effect to get going, but would it be possible to fund it without getting into the wholesale data harvesting and selling Facebook is doing?
Would a freemium model work? A less abusive advertising model? Charging businesses to be there, but not people?