If the user has already registered or connected to your site using Facebook, the autologin function should automatically log them in when they return if they are logged into Facebook.
One thing to know is that there is a timeout on the auto-login process to prevent problems with continuously logging users in. We use the Session Timeout setting in your Joomla configuration to set that timeout. So, whenever a user logs in through Facebook *or* when they logout of your site, the auto-login functionality will not kick in for the session timeout period.
To test, you should clear your browser cache (we use a cookie to set that timeout). Then, login to Facebook and go to your site and see if you are automatically logged in.
Hope that helps, but if you still have issues, just let us know!
Alex
Facebook Login Setting
