More than half of children in the UK using the internet have had an "unwanted experience", a poll suggests.
The NSPCC found 50.4% of 2,053 children had experienced problems such as bullying, being threatened or sexually harassed while online.
It is concerned about the popularity of social networking sites such as Bebo or MySpace, which it says 52% of children aged 11-16 use once a day.
NSPCC chief executive Dame Mary Marsh said: "Children face real threats."
The survey indicated most young people logged onto such websites to make new friends and link up with people they already knew.
But it also suggests that almost 60% of children used the websites to help combat loneliness, while 53% used them to share their problems.
The findings were revealed as the charity launches this year's Don't Hide It campaign.
The campaign aims to encourage children to speak out about all forms of abuse carried out on social networking websites.
Personal details
NSPCC director and chief executive Dame Mary Marsh said: "Children face real threats on the internet such as sexual grooming, cyber-bullying, exposure to violent, pornographic and other unsuitable material..
"Online social networking is part of millions of children's lives.
"We must recognise and respond to this reality by helping them be safer online as well as helping them speak out about abuse at the same time."
"Children face real threats on the internet"
Jim Gamble, Chief Executive of the Child Exploitation and Online Protection Centre, said an increasing number of children were putting their lives online.
He said: "The NSPCC research matches our understanding of the problems that children and young people are vulnerable to online.
"The rise in popularity of social networking sites, online gaming and chatting on instant messenger means that more young people than ever are sharing their personal experiences and details with strangers.
"In the virtual world the need for cautiousness is often forgotten. The warning signs traditionally associated with strangers are lost amongst the possibility and excitement of new relationships."
Previous research by the NSPCC has suggested 46% of children have given out personal details about themselves to people they have met online, such as photographs and phone numbers.
The charity's website advises children to never reveal personal details, including their real names.
Children worried about abuse can call ChildLine on 0800 1111 or visit www.donthideit.com for more information.
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