We missed it.  Foursquare celebrated its third 4sqday yesterday, April 16.  Get it – four-squared is 16.

According to a post on Foursquare’s official blog, the service now boasts 20 million users that have logged an astonishing 2 billion check-ins. High schoolers account for 13% of users and 21% of college students use Foursquare according to Ypulse research.

It’s a remarkable milestone for the service, which lets users “check-in” at various destinations and earn badges. Just last June, Foursquare used this detailed infographic to announce it had crossed the 10 million-user mark.