According to research, people aged 25 and older comprise 68 per cent and 71 per cent of the sites' user bases respectively.
An analysis carried out by research firm comScore Media Metrix has revealed significant age differences between the user bases of such sites.
Xanga has a younger user profile, with 20 per cent of its users in the 12-17 age range, about twice as high as that age segment's representation in the total internet audience.
Not surprisingly, Facebook, which began as a social networking site for college students, also draws a younger audience.
More than a third of visitors to Facebook are 18-24 years old, approximately three times the representation of that age segment in the general internet population.
"While the top social networking sites are typically viewed as directly competing with one another, our analysis demonstrates that each site occupies a slightly different niche," said Jack Flanagan, executive vice president of comScore Media Metrix.
"MySpace has the broadest appeal across age ranges, Facebook has created a niche among the college crowd, Friendster attracts a higher percentage of adults, and Xanga is most popular among younger teens.
"There is a misconception that social networking is the exclusive domain of teenagers, and this analysis confirms that the appeal of social networking sites is far broader."
Interestingly, an analysis of visitors to MySpace shows that the site has experienced dramatic visitor growth and has become more popular among older internet users.
The most significant shift has occurred among teens aged 12-17, who accounted for 24.7 per cent of the MySpace audience in August 2005, but today represent a much lower 11.9 per cent.
Conversely, internet users between the ages of 35-54 now account for 40.6 per cent of the MySpace visitor base, an 8.2 percentage point increase during the past year.
"As social networking sites have become mainstream, the demographic composition of MySpace has changed considerably," said Flanagan.
"Last year half of the site's visitors were at least 25 years old, while today more than two-thirds are 25 or older.
"It will be interesting to monitor the shifts in Facebook's demographic composition that will undoubtedly occur as a result of the company's recent decision to open its doors to users of all ages."
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