There's a few different things that can happen, depending on your configuration and the user logging in:
* JFBConnect can check if there is a Joomla user with the same email address as the one registering with a social network. If so, the 'connection' between the social network and the Joomla account is automatically made and the user is logged in.
* JFBConnect can redirect the user to a "Login or Register" page. On that page, the user can register a new account or login to an existing Joomla account
* When a user is logged into your site, you can have our SCLogin module show the social network icons for accounts they haven't already linked with. After they 'connect' to one of those social networks, they can login to your site later with that account.
I hope that helps explain, but if you have any other questions, just let me know.
Thanks,
Alex