You may want to pose this question to that addon developer in their presales questions page. I have not used that one before but I would imagine that they have a block that you can add to any page with the button or form for Facebook login. So you could probably just make any page, call it Login and then add that block to it. And then just hide any links to the regular C5 login. That would be easier than trying to edit the normal C5 login page which I believe is a single page.

Hi. Goldhat is completely right. You can disable registration in the Dashboard Settings and add a Facebook Connect block to your pages that allows login with Facebook.

Regards.
Nicola

So if I disable registration in the settings, registration is still possible? Good to know!

Hi. Sure (enlil sorry for my late reply to your icons, I'm working on the new release with them, thanks a lot!). If the block is present, registration is possible. This is due to many people using the block (me too) as the only possible authentication system.

This can be changed, for Social Login Package at
packages\social_login\models\social_login.php

and, for Facebook Connect Package at
packages\facebook_connect\blocks\facebookconnect_connect\controller.php
line 124, same code.
You can also change it to manual approval ($data['uIsValidated'] = '0';)

Regards
Nicola