While that approach obviously worked for Facebook, I think this is one of those areas where it's hard to pin down a "formula" that will work in every case without eventually having it be proven wrong in a big way. In this case, for example, it is conceivable (although maybe unlikely) that another major player such as Google or Facebook could disrupt Twitter by starting big. In other words, rather than targeting a niche until they gain mass, they could somehow leverage their existing mass to get ahead.