Brian Solis has a great article about the need for businesses to get onto location-based social networks. I suggest you read the full article, but some examples of brands using Foursquare are:
The Bay Area Rapid Transit (BART) partnered with FourSquare to encourage transit ridership
Starbucks offered $1 off any size Frappuccino for mayors and as a result, observed a 50% increase in check-ins at its locations.
Monique’s Chocolates in Palo Alto acquired over 50 new customers and earned over 100 redemption for it’s special of “buy one get one” for truffles. Each redemption also equal a 25% return ration. In comparison, the chocolate shop ran an ad in a local paper and acquired only one new customer at a cost of $300.
AJ Bombers, a popular burger joint in Milwaukee increased menu item purchases by 30% through its special promotions of free burgers for mayors and free cookies for adding tips. Also demonstrating creativity, AJ Bombers hosted the equivalent of a Tweetup for customers to help them earn Swarm and I’m on a Boat badges while increasing loyalty and sales.