Doctors at one of the country’s largest eating disorders treatment centres said they needed to act more responsibly.
The eating disorders charity, B-eat, said little progress had been made on combating “pro-ana” sites.
The networking sites said it was hard to distinguish between support groups and “pro-anorexia” groups.
But a spokesperson for MySpace said they were working with organisations such as B-eat.
Over 1.1 million Britons are known to suffer from an eating disorder.
The vast majority are women, although experts believe the numbers could actually be much higher as many cases go undiagnosed.
Specialists and charities say the rise of the internet and new media has played a significant part in providing easier access to information on how to get thin.
Research has shown that young women exposed to pro-ana websites felt more negative, had lower self-esteem, perceived themselves as heavier and were more likely to compare their bodies with other women.
Dr Ty Glover, consultant psychiatrist on the Eating Disorders Unit at Cheadle Royal Hospital, said it had proven difficult to shut down pro-ana websites, but the situation was slightly different for sites such as Facebook.
“Social networking sites can censor their material and we expect them to act responsibly,” he said.
We are horrified at the content of these sites and the tips they give on how to be thin.
“People with eating disorders are extremely vulnerable and often have very low self esteem, so pro-ana and pro-mia sites can be very damaging as they are sending out the wrong advice.”
It is thought there are more than 500 pro-ana and pro-mia internet sites.
Cheadle MP Mark Hunter has asked the government to promote awareness of the dangers of such websites.
“These websites are truly despicable and only help to reinforce the myth that having an eating disorder isn’t a problem,” he said.
Susan Ringwood from B-eat, who are due to launch a report this week calling for better support for families of people with eating disorders, said the websites encouraged people to avoid treatment.
“We believe that the sites should act responsibly.
“They have acted to remove other content that is seen as ‘dangerous’, or could encourage young people to do dangerous things.”
She said they were not calling for such sites to be banned completely, but for greater awareness of the dangers and for more support online so the sites are not the “only refuge”.
But Shannon Bonnette, who is recovering from anorexia, told the BBC that looking at anorexia web pages actually helped her overcome the illness.
“What I found through visiting those site was that there was a common theme – everybody stays miserable,” she said.
A spokesperson for MySpace said: “It’s often very tricky to distinguish between support groups for users who are suffering from eating disorders and groups that might be termed as “pro” anorexia or bulimia.
“Rather than censor these groups, we are working to create partnerships with organisations like B-eat.
“We have also placed ads on pro-anorexia profiles for up to a year from the National Eating Disorder Association to target these groups with positive messaging about how and where they can get help.”
A spokesperson for Facebook said the site supported the free flow of information.
“Many Facebook groups relate to controversial topics; this alone is not a reason to disable a group.
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Opening up about anorexia
Sources: BBC NEWS/HEALTH, 24Th. Feb. ’08