Thanks for a great post - it is very interesting to speculate and research in to how Facebook will use the blockchain, I agree that SMTs could be a reality (in the future!) for the platform.
I have been researching in detail to see what is possible for Facebook to make a move towards decentralisation - if you are interested I would be happy to do a post in which I play out the different scenarios that I think are possible. Facebook and other social media platforms do have a lot to learn from Steemit and the likes, but I think the key for Steemit is to make the platform more user friendly (purely from a UI perspective) to get more and more people onboard!