Facebook had a secret ingredient too, they were one of the only sites on the Internet to use exclusivity correctly. They built up hype and got users drooling by

Facebook had a secret ingredient too, they were one of the only sites on the Internet to use exclusivity correctly. They built up hype and got users drooling by slowing becoming available to more groups, and once they became available to everyone they were already established and validated. Of course luck and a simple design factored in too, but I think the exclusivity was the biggest part.