Ever since Facebook officially announced its Places location sharing service last summer, I’ve wondered how the company would fare against more established, but far smaller, location services like Foursquare.

Dennis Crowley, CEO and co-founder of Foursquare, announced a month ago at LeWeb that the service has surpassed five million registered users, a major accomplishment surely, but one we can’t really compare to Facebook’s 500 million registered users because the majority of those still don’t use Places. Though Facebook does update its Statistics page every so often, it has yet to add location data to the mix.

So, perhaps a bit unscientifically, I decided to compare check-in counts on both Foursquare and Facebook for the busiest U.S. airports. (Keep in mind that Foursquare launched in March 2009, while Facebook Places came late to the party, in August 2010.)

The quantity of check-ins on Foursquare are in almost every case well over double the quantity of check-ins on Facebook Places. On one hand, this could be a big bragging point for the small New York City startup, since this is a classic matchup of David and Goliath. After all, not everyone has the privilege of raising hundreds of millions of dollars from world-class investment firms like Goldman Sachs. On the other hand, David had over a year’s head start on Goliath; should the difference in check-in data be even more drastic?

Four months ago, some might have been surprised to learn that the launch of Facebook Places actually helped Foursquare have a day of record sign-ups, as more people turned on to the idea of location sharing networks. Based on Foursquare's recent five million member milestone and the data above, I'd argue that the "Facebook effect" is still significant, but will it last into 2011? Or, as location sharing goes more mainstream, will Facebook eventually squash its smaller startup competition?

While the purpose of my investigation was to just compare check-in data, I actually discovered something else intriguing: venue data on both Facebook and Foursquare is extremely fragmented. Searching for something like “Atlanta Airport” turned up pages and pages of businesses with that exact name, some with just one or two check-ins, others with hundreds or thousands. The story is similar on Foursquare, where searching for “SeaTac Airport” returned results as overly specific as “Sea-Tac International Airport Gate A1” and overly broad as “City of SeaTac,” but no venue seemed to be used by everyone for just the actual airport.

It’s great that users can just add locations missing from the network, but there should be some sort of better system that aggregates similar venues and cleans up all data. Honestly, maybe Facebook hasn’t announced any location data because it’s so all over the place, it can’t be properly calculated.

Either way, I eagerly anticipate an update from the company.