Today, the third annual 4sqDay (April 16, four-squared, get it?), check-in king Foursquare confirmed that it has reached new milestones of 20 million users and a whopping 2 billion check-ins.

Anyone who makes a check-in on the service today will receive the 2012 4sqDay badge, as well as the following message:

In 2010, foursquare fans declared April 16 4sqDay (4/4^2 – nerds after our own heart!). Two years and two billion check-ins later, you’re still why we get out of bed each day. Thanks to all 20 million of you for making us part of your lives. Happy 4sqDay!

Foursquare surpassed 15 million users back in December, and in March founder Dennis Crowley said it was close to 20 million users. We’ve asked the company for further comment on the milestones and will report when we hear back.

While it’s not growing as fast as uber-popular Instagram, Foursquare’s steady growth shows that there’s still plenty of value left in its check-in model, even if its revenue prospects are cloudy. The company stands strong while its biggest competitors, Gowalla and Facebook’s Places, are now out of the game.

The New York City-based company has raised over $71 million in funding so far. Last June, Foursquare landed a massive $50 million round with a valuation of $600 million.

Illustration: Sean Ludwig/VentureBeat

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