Foursquare had nearly 275,000 members by the end of January 2010

"Foursquare is primarily for letting your friends know where you are and figuring out where they are. Secondarily, it's for collecting points, prize "badges," and eventually, coupons, for going about your everyday business.
And it's becoming popular. Foursquare now has almost 275,000 users in less than a year."
Source: Business Insider, 28th January 2010
Update - Founder Dennis Crowley claims that they are nearly at 500,000 users