I just tried this as well on FB and couldn't. I copied some text and then tried to paste it into a reply to someones status. Nothing happens when I long press like it does in other apps. Looks like that option is not allowed in the FB app on Nougat.
I also just tried this on one of my S7 Edge phones still running MM and it allowed me to copy and paste in the FaceBook app. Looks like this is a problem with Nougat.