If you'd like to fix on your own for now, change that line in the controller (line 83, and another on 73) to enqueueMessage instead of enqueueError.
We're working on another RC release which will fix this and other problems. Unfortunately, when this is fixed, there is still going to be an error that the user is unable to be created (Which is what the error was trying to tell you in the first place).
Have you changed the configuration settings for the username creation? Most importantly, are you using the {rand} tag somewhere? Each Joomla username needs to be unique, so if you're testing with the same Facebook user to create multiple accounts, and not using the {rand} tag, JFBConnect will try to create a user with a username that already exists.. hence the error. This would also happen if 2 user's named "John Smith" signed up on your website.
Any other information on the user your creating would greatly help narrow this down.
Fatal Error
