Well how do you put a value on a place like Wall Street? It's not the number of people or the number of computers. It's what people do there and the amount of money flowing through. As someone said below, when you have this ability to datamine 18 million kids (in aggregate) then you're pretty much sitting on a mint. Every time one kid posts on another kid's wall, it's more data to feed into the system. College-aged kids are probably the most valuable demographic to marketers in general (not because college kids have the most money, but because marketers like marketing to them because it makes them feel young), and college-age kids who actually go to college are even more valuable because they have more money on average.
Hmm, I see your point. Value/Users does seem to be meaningless by itself. You could reverse it around and say $440 is how much value they think they can derive from each facebook user. But that does not take into account any future growth and as long as facebook stays *the* college social network it's going to continue tapping into that valuable demographic.
