Most likely, it's that the email address of the user you're trying to log in with exists in the user that was left behind from the usermap. If you're using Facebook User Only, or Full Joomla User with the 'hide email on registration' setting, then we have no way of getting an alternate email for the user. There are 2 options for this case:
1) Enable "Automatic mapping by email address" which, once the user authenticates through Facebook, they'll be automatically associated with the existing Joomla user
2) Make sure your template has the system messages tag so that you can see the error about not being able to save the user. This should be appearing in the case you describe, and if it's not, it's likely do to errors not being shown in your template.
Hope this helps, but if you have any questions, let us know!
Deleted account is not able to login
