If you signed onto Twitter last Friday, you may have noticed the rise of a formerly unknown hashtag: #4sqday. Believe it or not – this red hot topic had nothing to do with Justin Bieber, rather it marked one of the world’s first social media holidays: Foursquare Day.

Some of the world’s biggest brands, like McDonald’s, in addition to small businesses, like New York’s SideBar lounge, paid homage to the popular location-based networking service by offering Foursquare Day specials and throwing parties. In fact, events and specials popped up so quickly that the team over at Foursquare couldn’t keep up – and had to hold off populating their global map of #4sqday happenings.

Being social media super-fans ourselves, we couldn’t help but get in on the action by organizing our very own “swarm” (translation: event whereby 50+ users check in to a single location within an hour) at 360i’s Tweeting Bar to celebrate.

More than 100 mobile device-wielding employees, clients, friends and social media-loving randoms made their way to our twittering tap at 4:16 p.m. to check-in, grab a beer and score a special Foursquare Day badge. The amazing response demonstrated just how powerful tools like Foursquare can be when it comes to using online WOM to create offline experiences.

More About Foursquare

Foursquare allows users to “check in” to physical locations (bars, restaurants, shops and other points of interest) via their mobile devices to earn virtual – and sometimes real – prizes. Check-ins can earn you points of pride in the form of badges and mayorships, and even score you deals at some of your favorite businesses.

The location-based social networking service has enjoyed some serious momentum over the last few months, garnering nearly one million registered users and a handful of brand partnerships, including deals with Zagat, Bravo and the New York Times.

How did you celebrate Foursquare Day? Let us know in the comments below.