Facebook is odd. They were used to demo the reply from notification bar/center feature when iOS 8 came out. But was never updated to take advantage of that. And now there is no split view and no 3D touch either. It seems like Apple always uses them to demo features. But Facebook is always late implementing them.
I'm sure lots of app developers like Facebook, Youtube, Evernote and lots more will support Slide over and Split view in the near future Just give them time!!
I thought that is what the months of beta testing were for? Im just surprised that these big popular developers have not taken advantage of the new features.
Facebook app UI/UX itself is not very iPad/tablet optimized, I think they're focusing development effort more on iPhone and Android Phones, probably because they have a lot more active users on that platform.
It's probably true for many other major app developers too.
Since podcast support split-screen, it makes sense for other music player apps like Overcast to support it as well. Makes it easier when I want to select a podcast to play.
Chat apps like telegram haven't supported it as well.