Monetizing Foursquare

So I've been up on Foursquare for a few months now and have enjoyed it. I think it's a fun engaging way to get out and explore your city. I only have a couple of mayorships (one of them is my house!), and a limited number of badges, but I've enjoyed that aspect of it as well. With an active subscriber/participant base of a reported 1.8 million people, and an additional 20M raised in funding, it's apparent that other's likewise "like the Foursquare". 

My question is this. With as innovative a technology/idea as it is. What would you do, as Foursquare, to Monetize the business? They're not interested in hooking up with large retailers to allow them to "push" advertising to/on their users, there's only limited potential for traditional advertising on their mobile app, it only really works if you can continue to get large amounts of users which means allowing people to sign up for free, and in terms of how businesses get added and advertised, it's meant to be organic in nature. 

So my question to you is, basically this. What would you do as Foursquare to monetize the business? Would you even attempt to, or would you continue to build out the product and sell it off to a larger entity? Actually, that's a cop out and doesn't really make for interesting conversation even if that's what they're trying to do. At the heart of the matter is this. How does a company like Foursquare with a cool, innovative idea, that's getting tremendous reception, take and monetize their business? 

Inquiring minds want to know