In addition to Facebook registrations, your users can still register using the standard Joomla registration process. JFBConnect doesn't prevent or alter in anyway the normal Joomla registration process.
The login module we install with JFBConnect, the SCLogin module, is basically the standard Joomla login module with a few improvements, including the Login with Facebook button. The SCLogin and standard Joomla module both have links to the allow the user to register 'the old fashion' way as well as login with those credentials.
Hope that explains, but if you have any other questions, just let us know.
Thanks,
Alex