Parents who are desperate to find out what their children are up to in their spare time are joining networking websites aimed at teenagers.

Almost half of parents snoop around the sites that their children visit to check up on them, according to research. Parentline Plus, a helpline, says that calls from parents trying to keep tabs on their children’s internet activities are “regular and increasing”.

The explosion in popularity of social networking sites such as MySpace and Facebook, along with the messaging service MSN, have given parents a new headache, with many fearing that the sites are a hunting ground for stalkers and paedophiles. Others use the websites to gain an instant insight into their children’s private lives.

The Times writer Jane Gordon was outed by her daughter for registering herself on Facebook so that she could see her children’s entries.

“It has enabled her to see that my 15-year-old brother has listed himself on the site as being in an ‘open relationship’, that my little sister describes her religion as Amish, and that I was blind drunk at the weekend thanks to some pictures a ‘friend’ uploaded to my profile,” wrote her daughter Bryony in another newspaper.

Ms Gordon said that she was curious about what her children were up to. “Technology has created a big generation gap. In my day we had secret diaries, but they can have a whole secret life on the web.”

However, she said that her son had to help her set up the Facebook account and then gave her his password so that she could look at his entry.

“It all seemed quite sweet,” she said. “He is using it to keep up with friends from when he was young. I am worried that he is spending all his time on Facebook when he should be doing his homework.”

Jan Fry, of Parentline, said that calls to the helpline frequently ended up with parents revealing that their real concern was how much time their children were spending online.

“In many ways it is a general anxiety about children pulling away from the family, and a fear that the computer is beginning to rule their lives, although with others it is concern about whether or not their children are too young to be going online,” she said. “A few years ago, the age range was 14 to 15. Now it is as young as 10.”

Facebook has the largest number of registered users among college students worldwide, with more than 25 million members. It is the biggest internet picture site.

John Coleman, the former head of the Trust for the Study of Adolescence and now an Oxford academic, said that teenagers needed these sites in developing their identities.

“These sites are very much about the presentation of self, a really central aspect of adolescence. A lot can be disguised. They can play around with different identities,” he said.

Ms Fry said that it seemed to many parents that their children had secret lives online. Parentline Plus recommended that parents retain a sense of perspective, but be ready to lay down some rules.

“It is important for parents to remember how much time they spent on the phone talking to their friends. But just like with the phone, be prepared to set down limits, say half an hour each day after dinner,” she said.

And she recommends snooping if necessary: “If it looks like it is becoming obsessive, if your child is upset when they come off the internet, or if it looks like it is becoming a substitute for friendship, I would recommend taking action.”

A study by Sonia Livingstone, professor of social psychology at the London School of Economics and Political Science, has found widespread evidence of “covert monitoring” among parents of children aged between 9 and 17.

Her paper on internet literacy among the young found that 41 per cent of parents checked the computer to see which sites their children had visited, and that a quarter checked messages in their teenage children’s private e-mail account. However, she found that snooping was not guaranteed to help children to stay safe.

“Interestingly, if teens believe that their parents check up on them covertly, they encounter more, not fewer, contact risks,” she said.

Contact risks included visiting violent or pornographic sites or divulging personal information. “This might suggest that these teens react against the suspicion that their parents check up on them.”

But her research found that the more restrictions placed on children by their parents – in terms of instructions not to visit certain sites or to be wary of sites asking them to fill in forms – the fewer risks the children appeared to take.

She said that parents should make sure children understood the restrictions placed on them.

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