More than a million businesses use the Foursquare merchant platform

"Community: Over 25 million people worldwide
Over 2.5 billion check-ins, with millions more every day
Businesses: Over a million using the Merchant Platform (more information at foursquare.com/business)
Employees: Over 100 between headquarters in New York, an office in San Francisco, and a lovely outpost in London."

Source:  Data from Foursquare, 'updated September 2012', retrieved 5th October 2012