Half of secondary school children have seen sexual, violent and other adult material on social media, apps and games, a survey suggests.
The poll by the NSPCC also found more than three-quarters (78%) of youngsters reported that they had joined platforms before the age of 13, the minimum for joining a number of sites.
The findings came as the charity published an updated version of the Net Aware guide, produced in partnership with O2.
A total of 1,725 children aged 11 to 18 and young people, and more than 500 parents, were asked to review and rate the most popular social networking sites and apps.
The NSPCC said the top five sites where young people reported seeing inappropriate content were Sickipedia (100% of children who reviewed the site), Chatroulette (92%), Omegle (89%), ASKfm (88%), and Yik Yak (74%).
The research also found young people were more likely than parents to report seeing harmful content on three-quarters of the sites they looked at.
Peter Wanless, NSPCC chief executive, said: "It's vital that parents sit down together with their children regularly to talk about which social media sites they are using, and how to get help if they need it.
"More than 60% of young people we asked said social media platforms need to do more to keep children safe.
"These companies need to take more responsibility for keeping children safe online. We think there should be minimum standards in place and a new regulator may be required if industry can not regulate itself."
"Thankfully, it's not all bad news. Around half of the sites in Net Aware have made improvements to their privacy settings and reporting functions - a trend I hope will continue."
A spokeswoman for ASKfm said: ""ASKfm's top priority is the safety of users and we are working with others in the community, including the NSPCC, to keep teenagers safe online.
"ASKfm is not a place for abuse or hate, and we hold those who violate policies accountable by taking appropriate action based on each violation, which could mean anything from a warning to the closing of an account.
"The platform was rebranded in January to make it more engaging for users, with the safety centre at the forefront, so users can easily find information on how to block and report if it is needed."