I mean you need to pick your poison here. You either give facebook more data to validate your authenticity or you allow botnets to influence the social networks. It seems impossible to have it both ways, although I'm willing to hear alternatives in the general case.
Authenticity validation could be completely external from Facebook, and if they gave a shit about your privacy they would not only be OK with it but even push for it. Many countries already have crypto-secure solutions for verifying their identity when submitting things like tax forms. Facebook just needs to start using them, but they won't, and it's obvious why.
Because most countries don't have crypto-secure identity verification, which means Facebook would still need something like this? I suppose they could contract out to Experian (or other local providers) and ask you to answer some questions from your credit report, but I'm not sure that's really a good solution either.