First, make sure "Auto login Facebook Users is Disabled". Then, make sure the Logout Joomla Only setting is disabled, so the user will be logged out of Facebook too.
Then, I would honestly recommend that if the user is redirected to some special page after registering, you just add the following code:
<script type="text/javascript">FB.logout();</script>
That *should* auto-logout any user who is logged into Facebook. Obviously, a lot of testing will be necessary.
Hope that helps though, and let us know if you run into any issues!
Alex
Logout before login
