Facebook's audience is getting older, according to the latest figures from the company's iStrategyLabs advertising arm.

The data shows that users aged 55 and older grew by 513.7 per cent since January, compared to the 18-24 demographic that is prized by advertisers but which grew only 4.8 per cent.

The majority of the users on the site are now aged between 25 and 54, and this age group has grown by 190.2 per cent over the past six months to over 20 million users.

"The most troubling statistics we've seen are that there are 16.5 per cent less high school users, and 21.7 per cent less college users," said iStrategyLabs. "There have been rumours that these younger user groups are being alienated by their parents joining the service, and this data seems to prove it. "

It is possible that the current generation of users has graduated, and is moving on up the age range. But the statistics make worrying reading for the company.

Social networking sites like Friendster and MySpace showed strong initial growth followed by a dramatic tailing off. Facebook could be following the same model.

Overall Facebook has nearly 72 million users in the US, a growth rate of over 70 per cent for the past six months.