Alexandre,
I've used the virtual pages for Google Analytics in the past, but not for what you're looking for (and not with JFBConnect). I can't think of a 1-liner that will do what you're shooting for, but the below snippets should help. All changes are in the com_jfbconnect/libraries/facebook.php file.
First, find the "setInitialRegistration" function. Add the following to it, which will set that this has been an initial registration, so we can detect on the next page view (new user redirection):
$session =& JFactory::getSession();
$session->set('jfbcInitialRegistration', true);
Then, at the bottom of the initDocument function, directly below, add this code:
$session =& JFactory::getSession();
if ($session->get('jfbcInitialRegistration', false))
{
$session->clear('jfbcInitialRegistration');
$doc->addCustomTag('<script type="text/javascript">Call virtual Page JS here</script>');
}Code above looks good, but completely untested. Hope it works for you, but let us know if you run into anything,
Alex