By Jim Reed
Newsbeat technology reporter
Millions of young people could be putting themselves at risk by posting personal details online, according to a new report.
Half of all eight to 17-year-olds are signed up to a social network site
The telecoms watchdog, Ofcom, spoke to 8,000 adults and young people for the research.
It found that half of all eight to 17-year-olds are signed up to a social network site like Bebo and Myspace.
But a large number set their profile so that it can be seen by anyone.
James Thickett from the watchdog said: "Many young people use these sites for meeting new friends that they have never met before.
"That means anybody who has access to the site can have access to their profile. It could contain their name, their school, their hobbies and their interests."
Hannah, 14, told Newsbeat she has 200 friends on MySpace.
"I know most of them. Sometimes you get some strange people adding you so I just decline them," she said.
"But it's great to keep in touch with friends who have moved away and I don't see anymore."
Her friend Richard, also 14, said: "I use Myspace and Facebook to chat to my friends in my spare time.
It is just a bit of fun really but I do worry a lot about who I add. I first of all look at the pictures and ask if it's really a person I know."
The report also finds that a large number of younger children are using social networks.
According to the research, a quarter of all 8 to 11-year-olds have a profile.
Bebo, MySpace and Facebook set a minimum age of between 13 to 14 to use their sites but don't really check the age of their users.
That could change in the future though. The Government will publish a new set of guidelines later this week.
It is likely that sites will have to put in place stricter age checks.
Under-18 profiles will be set to private by default or only accessible by friends of the user.
Sites will also have to display the telephone number of the emergency services and support organisations like the NSPCC and Childline.