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.
The NSPCC says these could heighten children's exposure to abusive people.
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.
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."
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 http://www.donthideit.com for more information.
No responsibility can be taken for the content of external Internet sites.