Another bug with the Facebook login, it seems you are just sending the user back in history. So if I click on the login link from your OP, and then on the Facebook Login link, I get send back to this thread on facepunch.
Whoops, the login page sets where you end up after logging in, it's currently only set to the referrer, will fix.
I also get a "We could not validate your login" error, something I did wrong?
This I'm really not sure about, I've managed to login on two facebook accounts without issue (both re-logging in and registering fresh); does it happen every time you try to login?
Edited:
It *might* be fixed now, I've updated FD-Core with a more 'robust' (ie it follows the examples) facebook login; unfortunately I can't really test as it's working for me...