Foursquare users have checked in 3 billion times

"Foursquare has seen phenomenal growth over usage of its service. This week, it announced that it counted its 3 billionth check-in. Not bad for a service with just over 25 million users worldwide.
Two years ago, the location-based check-in service said it had experienced its biggest growth in history which was marked by more than 380 million check-ins, the Rally to Restore Sanity, and a single check-in from space. Now, 24 months later, the number of check-ins has increased exponentially — the company recorded its 1 billionth check-in September 2011, just 9 months later after its 380 millionth check-in.
Last September, the company stated that it had 2.5 billion check-ins counted. In that three month period, it averaged 167 million check-ins, or around 6.2 million per day. To get this number, TNW found the difference between November and September numbers and divided it by the days since (81) and that resulted in 6.2 million daily check-ins."

Source:  The Next Web, 21st November 2012

Earlier - 1.5 billion announced in February 2012