[TeeJay942]

The article says "Foursquare is on the path to $100 million in revenue", so they might be nowhere nearly that level. But I would think their largest costs are salary and servers.

Like most startups building a mass of users (e.g. Twitter), they have likely been optimizing for growth as quickly as possible. Which means spending all revenue/investment on getting customers, etc. and delaying focus on turning a profit until some future time (e.g., as the acceleration of growth slows).