Almost one in five children who use social networking sites suffered a negative experience last year, research by children's charity the NSPCC shows.
This included bullying, unwanted sexual messages, cyber stalking and feeling pressure to look a certain way.
The NSPCC also said a "large number" of users of Facebook, Twitter and YouTube were under the minimum age of 13.
A full report of the survey of 1,024 11 to 16-year-olds from across the UK will be published in November.
The survey also showed that the most common bad experiences among children were bullying and trolling.
This involves insulting or intimidating others, usually under a pseudonym, to provoke a reactio