IMO, Facebook did well for two reasons, brand recognition and phased roll-out.
After it started taking off at Harvard, you had the Harvard brand, which is a pretty big deal, to encourage uptake at other colleges. By going college by college, critical mass was easier to achieve, as it didn't need a lot of people, it just needed a lot of people at your particular college, to encourage the non-early-adopters to join in. By the time it was actually competing with MySpace, etc, for non college users, it already had a sizable user group in the key demographic, and as I recall, the "started at harvard" branding was still to be heard.
After it started taking off at Harvard, you had the Harvard brand, which is a pretty big deal, to encourage uptake at other colleges. By going college by college, critical mass was easier to achieve, as it didn't need a lot of people, it just needed a lot of people at your particular college, to encourage the non-early-adopters to join in. By the time it was actually competing with MySpace, etc, for non college users, it already had a sizable user group in the key demographic, and as I recall, the "started at harvard" branding was still to be heard.