Facebook has over 1.7 billion users, while Steemit has less than 100 000, so at this point Steemit would still need to get 1700 times more members to be as big as FB.
The sobering truth is Steem may only have about 10,000 active users and we don't know how many of those are bots.
Which can be a good and bad thing. Depending on hype, which is coming, whether I agree that it's warranted or not, Steem will be creating an economy based on tokens, that is monitored by bots and large whales.