The regulator's research found that 49% of 8 to 17 year-olds who use the internet have a profile on a social networking site such as MySpace, Facebook or Bebo. This is despite the fact that the major sites say that users under 13 should not register pages.
Ofcom also found that despite many parents' worries about inappropriate contacts through the sites and the publishing of personal information, the users of social networking sites are not very worried about privacy.
"Although the subject of much discussion in the media, in Ofcom’s qualitative research privacy and safety issues on social networking sites did not emerge as ‘top of mind’ for most users," said Ofcom's research. "In discussion, and after prompting, some users in the qualitative study did think of some privacy and safety issues, although on the whole they were unconcerned about them."
It found that 41% of 8 to 17 year olds with profile pages left their privacy settings to 'open', so that anyone could see them.
Reports have emerged that the Government plans to announce a code of conduct for social networking sites which would require them to set privacy settings automatically to stricter levels for users under 18 who sign up.
Last week child psychologist and television presenter Dr Tanya Byron produced a comprehensive report on children and technology which recommended exactly that measure.
"The incentive for signing up to one of these codes would be the opportunity for companies to promote themselves as responsible businesses with an interest in online child safety," said Byron in her report. "It is likely that the main consequence of breaching the codes would be public censure by the Council. Avoiding this kind of reputational damage would be a strong incentive for companies to co-operate."
Ofcom said that a large proportion of younger children have profiles. "27% of 8-11 year olds who are aware of social networking sites say that they have a profile on a site," said its report. "While some of these younger users are on sites intended for younger children, the presence of underage users on social networking sites intended for those aged 13 or over was also confirmed by qualitative research conducted by Ofcom."