Question using Facebook Login Button & Tracking


I was thinking about using a Facebook Login button as an option to login to my forum. I have a couple of questions.

1. Can I have both the Facebook Login Button and a regular login button through Xenforo, to give users a choice.

2. I saw on another site that they were offering a prize, like a free iPad, to the forum members who shared the forum with their Facebook friends. It worked like this. The forum users shared the forum with their Facebook friends. If say 10 of their friends signed up as members to the forum, the original user would get 10 entries towards winning the iPad.

My question is.. How does this work? How does the forum owner track the new members referred by the original user through the Facebook login. I know it has to be done through facebook login, but does anyone know how to make this work? Is this through Facebook API, Xenforo or ? How does the forum owner handle this.