I do not believe that Steemit is the Facebook killer like many are saying. Facebook is very different to Steemit. Facebook is primarily a way of keeping in touch with friends and chatting whereas Steemit focuses on posting stories with upvotes much like Reddit.
On Steemit, you can't add friends and you can't message them. Sure, you can follow, but that is completely different, because you don't need permission for a follow.
However, Steemit is just one of hopefully many social media websites that will run on the Steem blockchain, so who knows? Maybe an awesome Facebook clone will come out to compete with it that will run on the Steem blockchain.