It might be that i found a bug in this plugin.
When a user wants to login using FB and the email address already exists, they get the message 'User registration is currently not allowed.'
In the file class_wdfb_model.php inside method get_wp_user_from_fb().
On line 406 the profile info is loaded, this way it will only get the 'name' and 'id' (in my case).
On line 410 the if statement try to check for 'email' in the $me object, which isn't there (again, in my case).
from line 562 to 570 i see how the email is being fetched. Shouldn't that part be included in the get_wp_user_from_fb() method? I tried this and it works, but i'm not sure if there are some unforeseen issues.
if the above isn't the case, than the error message should be different.
I hope it's clear enough what i try to convey