My bet is some real-world tie, one which is time consuming and expensive to create. From there it should be possible to create moderation tools that keep the rest going.
An example of a real world tie would be a trust network that requires status with in-person communities and local businesses. And not just "accept the hot chick friend request," but an explicit "I'm staking my reputation by saying this person is real."
But once you let bots in, it's over.