Here is my solution.
I have created my own plugin to add tracking script to Login headers.
This way, the Login pages are also monitorized and I can add to Facebook the events asociated to specific URLs like "checkemail=registered"
This is myplugin.ptp content, replace XXXXX with your own codes:
Plugin Name: myplugin
Author: My Name * Version: 0.1
// Login page header
if ( !current_user_can('edit_posts') )
//Close PHP tags
add your code here between scripts tags
<?php //Open PHP tags
// Fin Header
/* Fin del cógido del plugin */
I have removed the rest of my pluggin to leave just the code related to tracking Facebook Ads. I hope I haven't make a mistake.
Create a PHP file named as the plugin (see "Plugin name" in the code) and leave it in plugins directory. Do not forget to activate it.