Seems like some have said that it is now: https://forums.macrumors.com/threads/facebook-for-ipad-pro-12-9-finally-optimized.2161069/The main facebook app isn't optimized for ipad pro 12.9.... still.
According to the article on 9to5, it's not an accident and was intentional because of having to use an older version of Xcode due to a crash for users on iOS 9.
I was looking at the pictures and wondering why I didn’t see this glaring horrible problem with resolution, but it seems folks are talking about the app layout? Honestly, I like it as it spreads the icons out a bit more as opposed to having the screen so crowded.
Just my humble opinion...