As much as I detest virtually everything Facebook, it's growth strategy from Harvard to Ivies to selective edus to all was great. From the get-go, it was the place to go to find a crowd more exclusive than the one you were in already.
That ended with mass-market availability, but it's provided steam to roll on for a while.
Google had an initial base which was, I think, the seed around which it could have grown a community but blew it by releasing publicly too early.
Twitter took off once it got a critical mass of celebrities using it, and this fact was published in mainstream media. Stephen Fry brought in a lot of people; now it's Justin Bieber.
