i dont think he can buy steemit. most probable adaptation of cryptos that could happen with facebook is facebook introducing two crypto currencies like
FBC - FaceBookCoin at 0.00001 and WhatsAppCoin - WAC @ 0.00001$
also for likes, posts and comments .. Thats it ...Done he will not just stand again rather he will become the worlds richest again for the next decade...
one problem they would be facing is with the previous votes, likes and comments. They can reset a few and verify every account again with their phone number ...