This is a great question, questioned a lot lately. I think they will attack platforms like Steemit, and I believe they are already doing it, but with small attacks, just because they are not feeling threatened yet. But when Steemit and other crypto-based platforms grow bigger, I think that FB will attack.
If not, they will make their crypto! It is the only way, except attacikng, that they can aontinue to be BIG...