A study this week suggests that while Facebook is growing in popularity, its users are getting older.

A report released by iStrategyLabs show that while the number of Facebook's US high school and college-age users declined over the past six months, its popularity among the 55-and-older crowd is booming. In fact, the number of 55-and-older Facebook users showed staggering growth - 513.7 percent - in the past six months, the digital consulting firm said.

This isn't a new phenomenon for Facebook. In March, Hitwise reported that the social network's audience of people over the ripe old age of 35 increased by 23 percent in February compared with the same month last year. While Facebook was first launched to serve college students in the States, over the past year or so the network has expanded to include many middle-aged people.

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While social networking sites were thought to be the domain of teenagers wanting to talk about school dances and their latest favourite band, it now looks like a quickly growing number of people on Facebook are closer to receiving their first pension payment than they are to taking their first college class.

The iStrategyLabs report also shows a 190.2 percent growth in the 35- to 54-year-old category.

Facebook isn't the only social network attracting older users.

A comScore, Inc. report released in April showed that people aged 45 to 54 are 36 percent more likely than other age group to use the Twitter microblogging site. That category is the biggest user of Twitter, followed by those aged 25 to 34, who are 30 percent more likely to Tweet out updates about their life and work.

And the news isn't just about older users joining social networking sites. Part of it about asking where the younger users are going?

The iStrategyLabs report notes that students are apparently fleeing Facebook. The report shows that Facebook has 16.5 percent fewer high school students on it and 21.7 percent fewer college students than six months ago.

"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," said Peter Corbett, CEO of iStrategyLabs in a blog post.

Overall, Facebook showed strong growth over the last six months, with the number of US-based users up 70.8 percent. And, ccording to iStrategyLabs, 54.6 percent of the social networking site's users are female.