Facebook is a popular social networking site, similar to MySpace. It was originally available only to college students, then high school students, and now anyone can join. Most groups at Facebook are formed by individuals, but companies like Activision can also pay to set up official groups for promoting their products.
You can access the Marvel: Ultimate Alliance group at this link (free registration or sign-in required). Click "Join Group" on the right to be entered into the prize drawing.