Foursquare has one million users

"Foursquare Labs Inc., the location- based Web service that reached 1 million users this week, will decide whether to sell itself or raise more venture capital within weeks, Chief Executive Officer Dennis Crowley said.
[...]
The 20-employee company offers free advertising, for now, to small shops and restaurants, while generating revenue from selling ads to chain businesses, Crowley said. Ultimately, it plans to offer targeted advertising for local businesses, by crunching data about where users check in to guess what kinds of offers they want to see, he said.
The company is rolling out new features, including a self- service platform to let small businesses create Foursquare ads, and may begin services to introduce users to each other, Crowley said. The number of users is growing 50 percent a month and is on pace to reach 3 million by the end of the summer, he said."
Source: Foursquare founder Dennis Crowley, interviewed in Bloomberg, 23rd April 2010
Earlier - 500,000 reported on 11th March