The registration page for wordpress is modified by the Ultimate Facebook plugin if it is enabled for registration. Seemingly undocumented (except the code within wpmu-dev-facebook/class_wdfb_public_pages.php) is that you must copy 2 files to your template directory for the registration pages to be templated properly.
The two files that must be copied to your template directory are:
Within these files, just below the "get_header()" section is an html area where you can include some custom code. As you may guess, registration_page_success.php is called after successful registration.
Hopefully this helps some people! I spent several hours tracing this down. This is great if you want to include custom messages to customers during the registration process, provide links after registration or even include up-sell data.
If you are using pro-sites, because the code within registration_page.php is hard-coded to forward to registration_page_success.php you may need to modify the formwarding link to pro-sites packages page to have customers sign-up.
Man WPMUDEV rocks! I'm having so much fun with this!