do_shortcode For "Register with Facebook" ?

Yes, can someone please tell me the do_shortcode for "Register with Facebook" .
Searched and searched and cannot find a solution.

Many Thanks, Brian Wallace

UPDATE: just viewed page source and figured it out.

<a class="wdfb_register_button" href="/wp-signup.php?fb_registration_page=1"><span>Register with Facebook</span></a>
But now I have one more question. What hook is it to not show if member is logged in already?

Once again, Thanks