I have been testing out the FriendCSV application mentioned on TechCrunch yesterday. FriendCSV allows you to download a .csv file with some - but not all - of your Facebook friend's contact information.

One thing I found interesting was the comparison between Facebook UserIDs and birth dates. It looks like Facebook userIDs are handed out sequentially, so you can get a feel for when someone's joined based on their userID.

My ID number is 500910058, so you can decide for yourself whether that gives me any Facebook cred.

To find your UserID number, click the Profile link and look for it in your browser's address bar:

http://www.facebook.com/profile.php?id=500910058

I plotted Facebook UserID's against the birth dates of my Facebook friends and ended up with the following chart:

Birth years run from oldest on the left to youngest on the right.

The lower right quadrant makes up the early adopters. But in the case of Facebook, the early adopters were also the only people with access to the site: college students. In my friend's case, this group is mostly made up of recent college graduates.

The lower left quadrant seemed strange until I spot checked it and found friends who were in grad school over the past few years.

I don't have an explanation for the gap between Facebook UserId numbering 200-500 million. Any theories? Did Facebook arbitrarily bump up their UserID sequence at some point? It kind of looks like they may have reset it to 500,000,000 based on the line of friends at that level.

Moving to the top quadrants, the far right is pretty light, since basically anyone of college age had already joined Facebook.

Things seem fairly well dispersed among age ranges - in my case, from the 1940's through the 1980's - over the time Facebook has been open to everyone.