Children as young as 12 are competing with each other to commit worsening acts of self-harm, a groundbreaking study reveals.
They described wanting to become "better self-harmers" and match horrific injuries they saw on Tumblr, one of the sites they chose because posts receive little scrutiny.
It is the first time researchers have been able to lift the lid on experience of such sites, after securing approval to interview young self-harmers.
It will fuel growing concern sparked by the death of UK teen Molly Russell, 14, who took her life after viewing self-harm images on Instagram.
The UK Royal College of Paediatrics and Child Health has called for urgent action by social media firms to tackle self-harm and backed demands for an independent regulator.
Dr Max Davie, RCPCH officer for health promotion, said: "We know that self-harm rates are high and rising, particularly among young girls, and so seeing the rise of accounts promoting self-harm is very concerning.
"The combination of social media's incentives to be noticed, and the lack of effective regulation, can be toxic and may be contributing to this rise."
The Cardiff University study found some young people only began self-harming because the internet provided a catalyst. Most, though, were already self-harming and went online "to make sense of their behaviours".
What they experienced online, however, largely normalised their harming so that it became "a routine, everyday activity", said the researchers. The children were also able to discover and share new practices and techniques.
"They became motivated to engage in further harm... the exposure to other individuals' severe acts made them want to become better self-harmers," the study reports.
One woman, aged 19, told researchers she was left feeling one small cut was "not nearly good enough".
The researchers discovered a "sense of competition". One woman, aged 23, said she chided herself when she saw images: "Why can't I do it like that?"
Tumblr was cited as the favoured site because it was easy to search and find images, enabled image sharing and was "not encumbered by the monitoring and intervention by other social media and microblogging sites", said the study. Instagram also featured.
Dr Nina Jacob, who led the research, said: "The lack of scrutiny and moderation, where you can purportedly 'do what the hell you like', together with perceived anonymity, meant the site was considered more authentic than alternative platforms."
One 19-year-old woman told researchers: "Kids as young as 12 can use it... and there's a big self-harm community on there. I got sucked into it and it did sort of increase the intensity of my self-harm again."
In the study approved by the university's ethics committee, the researchers displayed ads on 42,000 Facebook accounts, before 21 self-harmers - 18 girls and three boys - aged 16 to 24 volunteered for in-depth interviews.
Three quarters were attracted to sites that provided self-harming images. One described them as "triggering a rush like an addictive high".
Dr Jon Goldin, vice-chairman of the child and adolescent faculty at the UK Royal College of Psychiatrists, said: "When people look up the words 'self-harm' they should be directed to helpful sites which offer guidance and support, not to images of people hurting themselves," he said.
Tumblr refused to say how many moderators it employed but said it had teams to quickly take down any material that violated its rules by glorifying self-harm, and worked with charities and mental health experts to provide advice that automatically popped up when people put in self-harm searches.
Victoria McCullough, Tumblr director of social impact and policy, said: "Research has shown that deletion of content posted by individuals struggling with mental health issues can have the unintended consequence of ostracising them and preventing them from seeking out the support they need.
Together with government and advocacy leaders, we're working to develop innovative approaches that help those in need."
The Sunday Telegraph, London