Fixed it. I had to log in through Twitter, erase the Facebook account from OneAll, then link it to Facebook through the users screen not the login dialogue.
Okay, all the new users can get in, so there's at least that. When I go to Dashboard -> All Users -> my name -> edit, it shows no checkbox on Facebook. When I try to verify it that way it says "This social network account is already used …
Let's see... it is set to link to an existing account, so that much is good. I have an existing account with that email but it's still asking, so it looks like it's losing the email somewhere.