Fast support too.
Thanks.
After fiddling around I found a fix (or it got fixed by itself). I created a new user in Joomla, tried to log in with FB, didn't work, deleted myself, tried to login with FB and voila, worked again.
I think the reason why it 'broke' in the first place is actually my fault. I initially deleted the Joomla user and didn't realize I had to delete the same user from the component.
And after deleting myself the second time (sounds emo-is, hehe) I also realized that if you delete a user from the component it automatically deletes the Joomla user too.
About the JED review, I'll try it again when it's possible to do so. Thing is they've approved another review of mine which was submitted minutes after the InstantFBlogin review. Weird.