Does any existing kind of social network prevent that kind of attack? I don't believe so, and I'd think that's pretty difficult to accomplish. Aether does attempt to make this nontrivial, I believe, by requiring proof-of-work before posting things. I don't know, but possibly that proof-of-work also applies for impeachment votes (I hope it does!). Then depending on how hard that proof-of-work requirement is and how many people are already in the community, performing such an attack would become harder.