The genius of Facebook was getting students at elite universities to divulge lots of potentially profitable information for advertising. They did this because Facebook was an exclusive place just for elite college kids (and later just college kids). Now that its open to everyone it no longer has that value. I don't see how it can reasonably continue to increase in value (although they could certainly find a greater fool to sell it to).
I'm not sure of the experiences from other early facebook adopting schools -- but my old school (Bowdoin College) was among the first -- and it's still like crack to them. Not as ridiculous as a few years ago, but it's still wickedly popular. And from my perspective the alum retention rate, as the article mentions, is very strong.
I think the genius of Facebook is not just the eliteness, but instead a new word I'll makeup called 'localness'. Facebook cordons off the scary, wild web and presents you with people that you know or could at least see yourself knowing or bumping into. Combine that with early entry into the social networking world, very strong and appealing UI, and yes, eliteness, and a few years later, you've got 18 million users.
