Foursquare has had over 40m checkins

"Location-based social network Foursquare counted its 40 millionth checkin a “couple days ago,” according to a tweet from one of its developers.
The number shows that Foursquare’s growth rate is accelerating: Five weeks ago, the company announced that the total number of checkins had reached 22 million. In short: Foursquare appears to have doubled its checkin rate in just over a month."
Source: Mashable, 10th May 2010
Original tweet announcing 40m checkins
Earlier - 15.5m reported on 11th March