Foursquare's 15m users have checked in 1.5bn times

"Crowley said the 15 million global Foursquare users have checked in 1.5 billion times. With such a critical mass of data, the company is developing the ability to become the Amazon of locations: Serving up recommendations for places you would probably like to visit based on your profile and check-in history."

Source:  CNN Money, 29th February 2012