Sign in with Facebook - Username

Hi, I activated "Sign in with Facebook" and first Trial I set it to use e-mail as username. This did not work as the e-mail is appearing on the profile as the name of the user. I then switched to use the Fullname as a username. This was creating long usernames as it is not an ideal choice. However, it will be a great option if we allow users coming in from facebook enter and validate their prefered permalink name and next to it have a button to update their username with the Permalink name.