Every month or two, another tech startup enters the billion-dollar club.
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News stories dutifully report the amount of the round and the names of the investors, along with the new valuation number.
But a lot of times, a more fundamental question is left unanswered: How do these companies actually make money? Do they make money?
Here are the answers: The business models of 10 tech companies who haven't yet filed to go public but are reportedly valued over $1 billion -- along with one company, Foursquare, that's getting close to that number.
It's a mix of straight e-commerce, transaction brokering, and advertising.