More than half of American youths between the ages of 12 and 17 use online social networking sites, according to a new study from the nonprofit Pew Internet & American Life Project.

Older teens, girls in particular, are the most likely to have created online profiles at social networking sites such as MySpace.com and FaceBook.com. The study found that 58% of girls and 51% of boys in the 12 to 17 age group had created an online profile. Among those in the 15- to 17-year-old range, 70% of girls had online profiles compared with 57% of boys.

The reasons teens use social networking sites, according to the study, include maintaining contact with frequently seen friends (91%) and rarely seen friends (82%), social planning (72%), and making new friends (49%). Only 17% say they use such sites to flirt.

The phenomenal growth of social networking sites has caused parents and politicians to worry that posting personal information online exposes teens to online predators. But many of the young users of these sites appear to be aware of such concerns.

The study finds that 21% of the 55% of teens with profiles say their profile is not currently visible online. And of those with viewable online profiles, 59% say only their friends have access.

The Parents & Teens 2006 Survey involved calls to a representative sample of 935 teens from ages 12 to 17 and their parents. It was conducted between Oct. 23 and Nov. 19 by Princeton Survey Research Associates International on behalf of the Pew Internet & American Life Project.