By Michael Holden - LONDON (Reuters) - Children using hugely popular social networking Web sites such as MySpace.com and Bebo.com face bullying, unsuitable advertising and pornography, a report by a consumer watchdog magazine said on Thursday.

"Children are spending hours every week on networking sites, building up their profiles and chatting with friends. However what might seem like innocent fun has a darker, more sinister side," said Jessica Ross, editor of Computing Which? magazine.

Networking Web sites have taken the Internet by storm in recent years, allowing users to create their own Web pages, play games, exchange photos and video, and listen to new music.

MySpace boasts a global community of nearly 80 million users while Bebo said it had visits from two million British and Irish users in April alone, with over half aged between 13 and 18.

The Which? report said the freedom afforded to users on the sites meant they risked being abused by people claiming to be younger and also provided a "ripe environment for 'cyber bullying.'"

They cited one teenage boy's web page which asked his friends to vote on whether a girl they knew had AIDS and within minutes had also discovered links to pornographic sites.

"Parents may be shocked to discover that illicit teenage activity such as underage drinking and sex is out in the open for anyone to read," the report said.

The magazine's researchers said they had set up an account on MySpace, pretending to be a 14-year-old child, without having to any provide proof or age or identity.

"Teenage users need to be aware that there is no way of knowing who is behind the face of a 'friend,'" Ross said.

The report added that the mother of a 14-year-old U.S. girl was suing MySpace for $30 million (16 million pounds) after claiming her daughter was abused by a 19-year-old she met through the site.

Both Web sites have clear guides and advice, warning people to be careful about adding strangers to their contacts list and that harassment or inappropriate content should be reported.

Bebo's chief executive Michael Birch told Computing Which? that only a small minority of his site's users had encountered harassment, and the magazine said none of the teenagers it had spoken to had suffered any abuse.

Birch said children would behave exactly the same way as they would in the real world.

"Suddenly, parents are seeing this is happening and there's this knee-jerk reaction to blame Bebo," he told the magazine.

"Seeing what children are doing could help parents learn more about the realities of childhood."