Foursquare is a location-based social networking service who has just received its 100 millionth check in this last Monday evening. They announced this the next day on Tuesday. This check in has marked another landmark for the social networking service. Foursquare has seen a rapid increase in growth for the past few months.

On June 23, the company announced it was growing at the rate of a hundred thousand users for every tenth day it is in service. This rate would put its current user base at more than 2 million people if you assume that this growth continues to increase steadily over the past month.

What exactly is Foursquare?

Foursquare is a service that connects users with their friends and contacts in the real world by giving them the opportunity to register their current locations using check-ins. These check-ins can be shared with friends and sometimes even strangers via social networking services like Twitter. This can be typically accessed through a GPS equipped mobile phone. The users can also register new locations if their current locale does not exist in the database already of hundreds of thousands of places.

The company, however, did not detail where the 100 millionth check in took place in real time. Foursquare has just taken a $20 million venture investment that has turned the value of the social network at $95 million. For the preening “mayors”, their obsessive check-in in with Foursquare’s mobile software has been successfully monetized.

The company has managed to take in money before but not in this kind of scale and range. Foursquare’s original funding round in September 2009 reached $1.35 million which was followed by a presumably lower “bridge” round. Although the use of the location service has mushroomed, the interest of Silicon Valley’s deeper pockets also did as well.

This includes Yahoo which is believed to have bid close to $100 million for Foursquare in April. Facebook was reported to have held talks as well. Andreesen Horowitz is the venture firm which is putting in a current $20 million round which has also approached Foursquare in the spring before it decided to give up.

Foursquare Complaints Overlooked By Hype

Kara Swisher at All Things D predicted this acquisition last week. She says that the interest in Foursquare is overhypes and not very clear yet if that popularity translates into solid businesses. The fact is that it is not visibly clear if Foursquare will ever be refined into something genuinely useful for a considerable portion of society. This is actually possible if one has enough focus and iteration. It is even said that Foursquare is used mainly by urbanites to posture or even stalk private gatherings.

Still, over the past six months, the company has showed the public that it has the attention span of a drink-slamming club hopper. Maybe the new investors of Foursquare will be able to help focus and formalize the startup for a bit.