Another "Me Too!" post. This really sucks, but I'm sure it is something on the FB side since I don't think anything changed on the Apple side when it started. I was hoping today's update to MacOS 10.13.5 would fix it, but nope.
I've tried wiping the Internet Account for Facebook, and obliterating everything "Facebook" from Keychain to no-avail. I can't use the FB integration on the Mac at all, and had to start just posting things the old-fashioned say via the Web.
What is particularly annoying is Facebook keeps coming back in Internet Accounts- I'm assuming because it is stored in iCloud Keychain. It seems deleting it isn't really deleting it. Oddly- it works fine on my iPhone and iPad.