Chris,
Yes, after the first initial registration, when the user clicks the "Login With Facebook" button, it will automatically log that user in. We even have an option to automatically log Facebook user's in that have already registered on your site. That works if they're logged into Facebook in another tab and visit the site.
We don't have the automatic login functionality enabled on the demo site, but the mini-registration field and the returning login can be seen there as it is now:
www.sourcecoast.com/jfbconnect/demo
Hope that helps,
Alex