There's no way to selectively make Facebook users into one group, while everyone else goes into another. Here's some code you can change to do what you're looking for. This is all assuming you're using the "Full Joomla User" setting, if not, please let us know.
In the /components/com_jfbconnect/controllers/loginregister.php file, find and alter the following lines:
Line 43:
$newUserType = $userConfig->get('new_usertype');
Change to:
$newUserType = "Author";
And on line 62, change:
$user->set('gid', $ACL->get_group_id('', $newUserType, 'ARO'));
To:
$user->set('gid', 19);
Test these changes to ensure they have the desired effect!
Thanks,
Alex