Yeah, this is a bug we just recently discovered. It's been there a while, but was recently made more obvious since we added the ability to use the Joomla email confirmation setting (which blocks a user by default until they do so). The same effect happens if you manually block a user.
This will be fixed in an upcoming release. Because we have auto-login and a few other methods for a user to try to login through Facebook, we're working on the best 'generic' solution that will work across all login scenarios and profile plugins that also have the ability to block users (CB mainly).
Sorry for the trouble. If you have user activation enabled on your site (and not skipped in JFBConnect) or you actively block users, we'd recommend disabling the auto-login Facebook users setting for now. The other login mechanisms that can cause the loop are at least user-initiated, so it's more obvious why there's a problem in that case.
Login loop when user blocked in Joomla
