Unless you find a way to stop bots, they will overrun everything and all tag each other as "someone I've met"
lobste.rs has an account tree thing. I don't know how well it works at all, but maybe in a worst-case scenario the moderators can get rid of an entire subtree, at the cost of some collateral damage.
That's fine, it'll all be left-handed whuffie (in Cory Doctorow terms) because although they'll all tag each other, they won't be part of my tag web.