Facebook wasnt cool. The people on facebook were cool. I believe facebook took off for two reasons. One is the one you mentioned, that it was exclusive. Two was that it was the anti-livejournal/myspace. No css, no glitter, no strobing, no sound, no hard to read colors, no moving elements around the page. IT standardized where and what functions looked like, and acted more like creation software (photoshop) than the output of the document layout tools. Facebook was utilitarian in that you couldnt customize its look, you couldnt express yourself. It took itself as a white pages, not a portfolio.