All that it really was was a standard story of viral marketing, self driven by users of the community. Back in 2002, I created a viral site (more appropriately, ripped off the concept of another site and tweaked it a bit - somewhat like what happened in the social media space), and emailed it out to about 20 friends. 5 signed up (all in one city) and put their link out there, and the first month had about 500 visitors, followed by 5000 the next month, 50,000 the next, and about a year later, it had hit 18,000 visitors/day, with visitors from all over the US and internationally. The site has considerably died down now, but it's still getting a few hundred visits a day, and the top countries for users now include South Africa, US, UK, Canada, Philippines, and India. All of that from links from 5 high school students in one town.
Students from Harvard liked Facebook, so other students (their friends) wanted it, and it spread from there. Starting out with a specific group might not instinctively be the best way to grow a site virally, but you need to hit critical mass in one area before it can start spreading. All it was, was a slight variation on MySpace, just a bit more refined for the college crowd. No annoying backgrounds, emo music, and perceived privacy and exclusivity. Even if some of those aspects weren't there, just tweaking an existing idea a bit, and getting a new viral crowd to spread the site seems to work.