I very much doubt Mark will look to integrate with Steem.
These people are going to find it extremely hard (most likely impossible) to let go of the power in that centralised way that they have been coached to think.
The monopolistic oligarchy stuff and all.
I guess Facebook are going to look to do something alongside how Ripple works.
A centralised crypto token which is still under control by Zuck by and large, but gives Facebook people a 'sense' of ownership while not really giving them anything of real value.
Remember, Facebook has a huge 'backlog' of centralised datacenters which, if Facebook was to be operated in a decentralised manner, would have to be ... dismantled.
This is a huge job for them to battle with this 'counter' infrastructure.
There is also another very important aspect to all of this especially when it comes to Steem itself.
It's not clear as to how centralised or decentralised Steem in itself is.
One could argue that Steem is way too complicated for regular people to understand how the rewards work.
I certainly struggle with this even though I'm theoretically a very smart person in comparison to majority of the populous.
Facebook has to make sense to the lowest common denominator people.
If it doesn't, things simply wont work.
This is all to do with #UX #Design and things like that, which is something I've been an expert in for 10+ years now.
I hope this helps to explain it and cast a bit of well educated light on this matter.