Which login is redirecting to the old URL? Is this when you do a social login, through Facebook and/or Google+, or just logging in with a standard Joomla user?
JFBConnect doesn't store your domain anywhere that should be making the redirect go to the 'old' page, so you may also want to make sure you don't have some redirect setup somewhere else that could be causing problems. That could be in your .htaccess file, an SEF extension or something like that.
Keep me posted and we'll help how we can.
Thanks,
Alex