If Foursquare Thinks It's Worth $500 Million, Where's the Revenue Model?

Check-in service hasn't proven it can make money

Foursquare is in a bit of a bind. The location-based social networking service is looking to raise a reported $20 million to $40 million for its next investment round, based on the idea that the company, which has 8 million users, is worth as much as $500 million. The problem? Foursquare has yet to demonstrate a successful revenue model—or even show that its principals care much about coming up with one that can justify that kind of valuation.