New York—Foursquare, the mobile service that combines social networking with location, has raised $50 million in a third round of venture funding. It plans to use the money to increase staff. According to the company, Andreessen Horowitz led the new round, with current investors O'Reilly AlphaTech Ventures and Union Square Ventures. A new financier, Spark Capital, also invested in this round. Last week, Foursquare partnered with American Express Co. to launch a location-based, couponless offers service to AmEx customers, adding to the rapidly proliferating local-deals landscape.