The problem with location-based social networking sites/apps is that today, almost all social networks became location-based… Therefore, in order to establish a unique name and continue growing in this niche-no-niche, the site/app really has to offer some unique experience through extraordinary features.

Apparently, Foursquare succeeding achieving this goal.

Today, Foursquare is celebrating for the third time its yearly 4sqDay (explanation below). To mark this day, anybody who will check-in today will be awarded with a special badge and a message that indicating on some very interesting statistics- Foursquare has 20 million users and an overall astonishing number of two billion check-ins since the service went live in 2009.

Geeky explanation for those of you who are cool enough NOT to understand why this day is called “4sqDay”- The sum of four squared is 16 and today is the 16th of the fourth month of the year, April. Here’s the message that appears when users check-in today and the badge:

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!

Although Foursquare growth isn’t as explosive as other niche social networking sites such as Pinterest which became the third most popular social network or Instagram before and after being acquired by Facebook, the check-ins social networking site still shows constant steady growth.

Slightly less than a year ago on June 2011 the company has announced on hitting 10 million users and now it gathered another 10 million. Not so while ago, this kind of growth would have sound almost impossible but today it just strikes as a nice achievement.

What does impressive is the ability of Foursquare to remain relevant and unique in a world where essentially almost every social network offers location or check-in features of some sort. Interesting to see if Foursquare will manage to stay a standalone growing service or it will eventually share a similar fate to Gowalla.

If indeed there will be some company who would desire to “check-in” Foursquare, it won’t come cheap- A year ago, when it was half the size as today, the company raised a capital with a valuation of $600 million.

In any case, here’s a great video released by Foursquare to mark 4sqDay: