Facebook's early growth was based in large part on tactical open-ups of the service. They started with just Harvard (where I'd say the "who's got the most frien

Facebook's early growth was based in large part on tactical open-ups of the service. They started with just Harvard (where I'd say the "who's got the most friends?" game kicked things off), expanded to the Ivies and then elite non-Ivies (who desperately want to be like Harvard) and slowly, slowly worked their way down the college eliteness scale, adding similar types of schools at the same time (northeastern LAC's in one clump, west coast state schools in another, etc). Schools who weren't added yet wanted to be in the elite club, and schools that were already in were too busy editing other people's wall posts to notice the elite factor of the site diminishing. Eventually they opened it up to all colleges, but that was probably well after they had opened it up to the colleges that actually mattered, numbers wise. The college growth sorta has buttressed the rest of the growth, since it pulls in new users as they go to college and retains old users as they graduate. Meanwhile, the site's features progressed along, making the site increasingly sticky (it really wasn't back in the day). The big hitters were and still are Photos and Events, which both happen a lot better if all of your friends are already signed up for the site. All of this stuff happened well before there was a lot of hype from the Valley started kicking in. Facebook's growth since opening up to high school (and later, the world) is less interesting though, since they've basically been running on network effects, inherent virality and quality product. They didn't have any of those when the site first started.