I imagine that if any blockchain-backed decentralized synchronize database messaging system ever got close to being taxed by carrying Facebook-like activity, we'd see migration to a newer, faster – or at least better backend-supported database solution.
That is, after all, what we're talking about – a distributed database with replication which covers keeping track of all the usual things that a forum/social network has to keep up with as well as token transactions.
Or, more succinctly, "if that ever seems to be a threatening situation, I feel pretty secure that some solution will be found by then, because it's going to be a long time before that's an issue."
If we even get close to such activity, Steem makes it completely trivial for the top witnesses to come together to decide to increase the block size.