I have the same problem. Two days ago it worked fine in the morning and when I tried to use it that evening it stopped working. Same error message. I've deleted my FB account from he system preferences - internet accounts and re added it numerous times. I updated to High Sierra. Still no luck. I have the same problem with other apps such as Luminar which also have the "share to Facebook" feature. Interestingly, in Luminar, the sample photo will share to Facebook no problem but my own photos will not. I was on the phone with Apple for more than an hour yesterday. They feel it's a Facebook thing that needs to be worked out on their end. They said they were able to replicate the problem on a lab computer and suggested in the meantime I should just drag photos to a folder and then upload through Safari. I'm not convinced a Facebook error is the reason because I've gotten pop ups asking for my FB password several times, even though I've put it in and it seemed to accept it as valid when I set it up in the preferences. When it randomly asks for the password (as a pop) and I try to put it in, it won't accept it. It's driving me crazy. I hope FB fixes this soon!
One other thing to note, I can post to Facebook through the notifications center. You can't add pictures there but you can post a comment and that works fine. That also works because the account was added through the system preferences > internet accounts.