As a very, very early user of Facebook, one of the big appeals was the exclusivity of it (way before games). You couldn't sign up unless Facebook was available for your university and you had a university email address. This legitimized it more than say, MySpace, where just anybody could sign up. They started with the "right" demographic, I believe (college kids). Because once FB became a thing at colleges (and to the exclusion of everyone else), it created demand from teenagers and high school kids as the cool thing college kids did. It created demand from college-aged people that weren't going to university because that's where their friends were. Then eventually, it started creating demand in what we then considered weird places, e.g. parents and grandparents, and businesses, who started showing up because that's where their kids were going. I always hear companies talk about teenagers as the most important target demographic, but I think Facebook (perhaps inadvertently?) discovered the "best" demographic, since it automatically attracted people outside of that. I dunno, just some thoughts.