Facebook may have begun as a network for college students, but if new demographic estimates are correct, the social networks’ audience is skewing much older.

According to iStrategyLab, the overall number of users between 18 and 24 years of age has grown
only 4.8% between January and July of this year. That’s compared to a 513.7% growth among users older than 55.

Of course, more high school and college students use the site than 55 year olds, so there’s less room for growth. But if younger users don’t replace current users as they age, it could be very bad news for Facebook.

IStrategyLab found the numbers by using Facebook’s own demographics in the site’s self-serve advertising program, which Facebook says are very rough estimates.

In addition to those growth numbers, they found that usage amongst 25 to 34 year olds grew 60.8% and usage among 35 to 54 year olds grew 190.2%. But most surprisingly, the number of high school and college students actually declined by 16.5% and 21.7%, respectively. (The full chart is below.) If the numbers are accurate, that may just mean that users are no longer
identifying themselves by their school. But for a site that hopes to collect and store all personal information online, even that is a problem.

A more disturbing trend would be that younger users are going elsewhere to share information. Older peple are still flocking to the site to catch up with old friends and explore the network. This year, Time even had an article titled “Why Facebook Is for Old Fogies.”

But getting people to use the service for more than just catching up with lost friends is the secret to Facebook’s survival. Many other social networks have stumbled after reaching a saturation point for network connections. Facebook has been working hard through Facebook Connect and OpenID to make itself a sticky site that users interact with frequently online.

If users stop going to the site to keep their friends and acquantances up to date on their lives, Facebook’s ambitious advertising plans have little chance of turnin a profit.

image: SheConomy