If you’re having problems getting the facebook login callback to fire – for example, you’re using one of the social plugins and are not getting redirected back to the right place on login – you might be looking to set the “Post-Authorize Redirect URL” – except, it’s not there any more!
Facebook seems to read this from the canvas URL now instead – so try setting that to be the same as your Site URL.
Incidentally, I only had this problem when working from localhost – with sites on the public Internet, setting the site URL only worked fine.