Hmm.. I just tested doing a language string override, hoping that would help. Unfortunately, even if you make the string blank, an empty 'system message' box will still show. That would have been ideal, but not possible.
What you'll need to do is modify the /components/com_jfbconnect/models/loginregister.php file. In there at line 108 and 265, you'll see the following line:
$app->enqueueMessage(JText::sprintf('COM_JFBCONNECT_MAP_USER_SUCCESS', $provider->name));Delete it and the message won't appear on new registrations.
I hope that helps get you going, but if not, just let us know.
Thanks,
Alex