Social networking sites such as Bebo and MySpace must do more to police what users do, warns Computing Which?
Researchers found it easy to pose as teenagers
A study of the sites by the consumer watchdog unearthed pornographic images, evidence of bullying and inappropriate adverts.
It also proved easy to pose as a child as the sites did no age or ID checks.
"Teenage users need to be aware that there is no way of knowing who is behind the face of a 'friend'," said Jessica Ross, Computing Which? editor.
During its investigation Computing Which? took a detailed look at Bebo and MySpace which are among the most popular social networking sites for teenagers.
In recent years sites such as MySpace and Bebo have built up a huge following. MySpace claims to have more than 80m users.
The sites let users chat to each other and give each one a personal webspace that can be used as a blog, illustrated with images or to host video clips.
The study found a "sinister" side to the social sites and revealed that the ease with which accounts can be created left open many avenues for abuse.
One researcher was able to pose as a 14-year-old on MySpace as the site did not require identification or carry out an age check.
In one incident of cyber-bullying one teenage boy organised a vote on his page on whether a girl had Aids.
Also only minutes after signing up for the sites the researchers uncovered links to pornographic images.
MySpace took a couple of hours to remove one image once it was reported but on the Bebo site a pornographic image remained two days after it was first spotted and reported.
The sites only monitor images after they have been posted and, said Which?, rely heavily on users to report abuse.
Ms Ross said: "Networking sites, such as myspace.com and bebo.com, need to do more to protect their millions of trusting members."
MySpace and Bebo employ security and safety officers who trawl the site for inappropriate content. Computing Which? said the teenager users of the sites it talked to knew how to block unwanted communication and to avoid distasteful sections of the sites.
In response to the report Bebo boss Michael Birch said: " Sadly, children face risks in everyday life, whether playing in their school grounds or on the internet. We are very conscious of this and are working hard to ensure our site is not abused."