The ability to create a Joomla account along with the Facebook account has been a part of JFBConnect for over a year. There's an option to create only Facebook users (no Joomla Username/password, so the user always has to log in through Facebook) or to let them set their username/password during registration. We use the Facebook only option on this site, cause we want people to register and get on quickly as possible (less hassle/forms before a purchase). We use the Full User option on our demo site to show how that feature works:
demo.sourcecoast.com/
For more information on these 2 settings, check out the "User Creation" settings in the configuration guide:
www.sourcecoast.com/jfbconnect/docs/configuration-guide 
Custom integration
You might also want to add an option for a password along with a username. This way they would have a regular Joomla account along with their facebook account. The reason for this is that sometimes the link with facebook can go down or act up and then the user cannot login into the site. But if they had a credentials as part of a regular account they could. 
You just sold me! 
