by Claudine Zap If you hit your favorite café, nose around a new neighborhood, or visit an out-of-the-way place, and nobody knows, does it matter? Well, it should. That's the thinking behind Foursquare , a social networking site, game, and user-review site rolled into one. And it's behind a race to the North Pole that has pit a 15-year-old Eton student with a 44-year-old CEO of an insurance company. Here's the deal. Foursquare, which has about half a million members,