Webatelier,
Still not sure why the blank string is showing up, but the below code hack will help you to remove the call altogether. It's best if you could do it through the language file, as this change will be required any time you update, but at least it will get you past this for now.
In the /components/com_jfbconnect/libraries/facebook.php, around line 204, you'll see:
$app->enqueueMessage(JText::_('COM_JFBCONNECT_AUTOMATIC_LOGIN'));Simply put 2 forward slashes in front of this line, and it will comment it out so that it doesn't even attempt to display that string.
Hope that helps, and good luck,
Alex