Facebook has been around for almost 15 years now. Of course there will be many many many more users on that platform than there is on steemit. We should give steemit another 5 years to let it fully bloom first.
Also, I am no expert on blockchain technology, but even if facebook decides to create a blockchain, how are they gonna get all this existing accounts onto that blockchain because these accounts were not created on a blockchain to begin with. And even if there was a way (there might be, I don't know coz as i said, I'm no expert) that's not going to be something you just decide to do and do it!! We're talking about billions of transactions being attempted to be processed at the same time and this has realistic risks to collapse even strong and established blockchain.
Steemit has an advantage here that all our accounts were already created on this blockchain and so are all the newly created accounts. We've been doing it gradually over time, which already puts us a couple years ahead of facebook if they were to implement an blockchain.