I think Facebook will still stay for a bit, because of Network-Effect. Furthermore many people are not familiar with STEEMIT, or in general with Cryptocurrencies, yet.
BUT: All contents which really bring value to people will move to reward-Plattforms like STEEMIT. It is just a matter of time.
ON STEEMIT I earn ZERO in the worst case. On Facebook I earn ZERO in the best case.