Analytics company iStrategyLabs has examined the demographics stats from Facebook's Social Ads platform, and they've reached some very interesting conclusions. Facebook's userbase, as a whole, is getting much older very fast.

As you can see in the chart below, the overall number of users between 18 and 24 years of age has grown only 4.8% between the fourth of January and the fourth of July of 2009. In comparison, the number of users aged 25 - 34 has grown 60.8%; the number of users aged 35 to 54 has grown 190.2%, while the number of users older than 55 years has grown a tremendous 513.7%.

If the iStrategyLabs numbers are correct, Facebook, simply put, is not a young site anymore. Most of the users (20,3 million, or 28.2% overall) on the site belong to the 35 - 54 age group. Compare that to the age group that was once Facebook's bread and butter - the 18 - 24 group - which is now in third place with 18 million (25.1%) users, behind the 25 - 34 year old group, which makes for 25.2% of Facebook's user base with 18.1 million users. The number of users aged 55 and over has grown from negligible 950,000 to 5.9 million in mere six months.

Now, although it may seem like the number of young users has declined, this is not true. The overall number of users of all ages is growing. But they are growing at very different speeds, and therefore the percentages have changed significantly; on a site like Facebook, which lives on advertising, this is a big deal.

However, although the number of young users has increased, the number of high school and college students has declined by -16.5% and -21.7%, respectively. This can indicate several things: first, that the data that iStrategyLabs is incorrect or very rough (which is a possibility, since Facebook doesn't guarantee that the data provided to advertisers is absolutely accurate); secondly, it's possible that Facebook users simply don't think that their education, or the school/college they're in are very important so they're simply not entering the data. It's probably a little bit of both, but it'll be interesting to see and compare Facebook's own demographics data with these numbers.

In any case, these are significant changes. If you show the same ads to Facebook users now, they will react vastly differently than they would have half a year ago. If you're an advertiser on Facebook, you should take these changes into account and react accordingly, because your campaign might not be as effective as it was a couple of months ago.