For those of us who didn't know anything about facebook, here's the wikipedia entry:
Facebook, formerly known as thefacebook, is a social networking service for high school, college, and university communities, primarily in English-speaking countries. The site has some similarities to MySpace, but differs in account availability, user control of display content, real-world identity, and overall neatness of appearance. As of December 2005, it has the largest number of registered users among college-focused sites (at over six million US college student accounts created with an additional 20,000 new accounts being created daily).
Anyone with access to a valid e-mail address from 2,000+ universities can register for and access the site, a group that includes students, alumni, faculty, and staff, although the vast majority of Facebook’s users are students. Facebook is also available at 25,000+ American high schools. The site is free to users and is financed by advertising. Facebook employs people in Palo Alto, California and in Boston, Massachusetts.