Many teenagers and younger children are signing up for social networking sites says telecoms regulator Ofcom in a report.
In a research exercise covering 3,000 children it found that about a quarter of those aged between eight and 11 have a profile page on sites such as MySpace, Facebook and Bebo.
This is despite the fact that these sites have a self-imposed minimum age limit of 13 or 14.
The news comes soon after the release of the Byron Review which investigated the risks children faced from exposure to harmful or inappropriate material on the internet or in video games.
At the same time the Home Office is preparing to issue recommendations on the steps social network sites should take to make themselves safer for children.
BBC News looks at the issue and what parents can do to keep up with their children and ensure they stay safe online.
What has Ofcom found?
The telecoms regulator carried out an in-depth study of how people, both young and old, use social network sites.
On these sites, which includes destinations such as MySpace, Facebook, Bebo, users maintain a webpage or profile about what they are up to. This can include blog or journal entries, pictures, videos and other information about their daily lives.
CEOP online safety tips for parents
Know what your children are doing online
Get them to show you how to do things
Help them understand not to give any personal information to online friends
Teach them to ignore spam
Teach them to ignore files sent by people they don't know
Teach them some people lie online
Tell them to keep online friends online
Keep talking so they know they can always tell you if something makes them feel uncomfortable
Show children how to block people online and how to report them
Most allow users to invite people they know to join the site and become part of an online group of friends.
In recent months social network sites have become enormously popular MySpace, for instance, has many millions of dedicated users.
Ofcom talked to 5,000 adults and 3,000 children and found 49% of those aged between eight and 17 have a profile on these sites. As noted above many children with profiles are below the minimum age set, but not actively enforced, by the sites themselves.
Perhaps worryingly for parents 41% of the children Ofcom questioned has their profile set so anyone, rather than just friends could view it.
At the same time the Byron Review warned of a growing "digital divide" between parents and their net-savvy children.
Why is this a problem?
The combination of children sharing sensitive data with anyone who cares to look and parents who do not monitor what their offspring do online could be a recipe for disaster.
CEOP online safety tips for children
Don't give your real name on gaming sites
Best not to have anyone on your IM (instant messaging) list that you don't know in the real world
You can block people in IM and chat areas
Best not to meet people you meet online, they might not be who they say they are
Tell an adult you trust if an online friend asks to meet you
Report a contact to CEOP if you think they might be an adult
Many children have received unwanted approaches from paedophiles while using the net. Figures quoted in the Byron Review suggest that 31% of 9-19 year olds who use the net weekly have received sexual comments via e-mail, instant message, chat or text message.
Research by the Child Exploitation and Online Protection centre (CEOP) suggests that chat rooms and instant messages are the preferred route for such contact but social networking sites are becoming popular too.
These dangers should not be exaggerated. It is still the case that very few abusers of children are strangers to their victims.
Are there other risks?
Publishing sensitive and personal information could leave people and families open to fraud and identity theft.
Unsupervised children might also open messages which harbour viruses or visit booby-trapped sites that infect PCs with spyware.
Children could also be at risk from so called "cyber bullying" in which online sites are used to mock them or they are bombarded with nasty messages.
What is being done to tackle this?
Many websites, not just social network sites, discourage children from registering but the restrictions are often easy to circumvent.
Few sites actively police these age limits though many will remove content if it breaches their codes of conduct.
Some places, such as MSN Messenger, also feature an icon via which children can report when they see or suffer inappropriate sexual contact.
On 4 April the Home Office is due to publish its guidelines for social network sites and what they can do to make using them safer. Currently there are no laws governing how children can set up profiles on social sites.
The government has also been behind many campaigns that aim to educate children about the dangers of using the net. Europe also runs the Safer Internet Day campaign which targets schools and runs events to get children thinking about ways to stay safe online.
Other places to look include ThinkUKnow, Get Safe Online and the Internet Safety Zone. Many publish easy to understand advice guides that help explain the risks and actions that can be taken.
What can parents do to help?
A lot. Every report on net safety stresses that parents have a huge role to play in monitoring their children and educating them about responsible web use.
One of the best ways to keep an eye on what a child does online is to site home computers in family areas rather than in their own bedroom. Children are far less likely to indulge in risky behaviour if they know others can see what they are doing.
Many organisations recommend that parents get much more involved in what their children do online. One good tip is to get a child to explain what they are doing and teach their parents at the same time.
Parents should also impress on children that they should ignore spam, be suspicious of anyone sending attachments via e-mail and warn them that many people lie online and may not be who they claim to be.
Technical measures, such as filters and security software, can help but none are 100% effective. Parental involvement and monitoring are just as important.
Finally, parents also need to realise that even though children have a greater understanding of the risks that does not mean they stop taking risks.