Well yeah, I'd first like to know how one can grow such an army of bots in the first place. Wasn't there a problem with fake FaceBook (bot) accounts as well?

I think you cannot prevent the creation of bots/accounts since there has to be a low barrier of entry but Proof-of-Brain is definitely something to look at because now it's just an term without meaning. Proof-of-Brain ≠ Proof-of-AI or bot