A truly open social graph, distributed so it isn't controlled by a single entity, is a game changer. It lowers the barriers to vertical oriented social networks, which wouldn't have the barrier of reconstructing every user's social graph.
The trick is the transition. It's like we are stuck in a local maximum. Once we move to distributed social graphs we are obviously better, but I can't imagine how this transition would occur, other than Facebook itself having an amazingly long term vision and going all in with this vision. There's a lot to be gained from being the industry leader for 50 years instead of monopolist for 10...