I've recently tried using FB via TOR (Browser) for the first time, but was unable. After entering the onion address and my FB credentials, I was informed that the account is temporarily blocked (presumably because of first access via TOR). I was presented with an option of unblocking it by recognizing a few photos of friends and matching them to names - but unfortunately, all those photos showed as blank, white squares!
So, I wasn't able to login via TOR via the purposefully created .onion address. Also, sent an issue report via non-TOR login about this, but never got any response.
Note also that this seems to mean to me, that there may be people who are cut off from FB via TOR same as me, but who don't even have a way to notify FB about the fact. And thus not having any chance of having the bug fixed.