Mathematically-speaking, four squared is 16, so 4/16 — as in, April 16th, which is today — has been Foursquare's official "4sqDay" for the past few years. This year, CEO Dennis Crowley and his team decided to do a video in honor of the occasion, which provided a platform for boasting about a few nicely rounded numbers: 35 million places, two billion check-ins overall, and a claimed 20 million users (it was estimated at 15 million back in December). That latter statistic, as with other services, always requires numerous caveats — Foursquare didn't say whether those 20 million were active users or folks who had just signed up at some point in the service's lifetime and may no longer even have the app. But that's not what's important right now: what is important is that you remember to check-in somewhere today and get that 4sqDay 2012 badge. Offer expires on April 17th, obviously.