Posts on Twitter and Instagram often promote "bonespiration" and "thinspiration" - women's bodies featuring protruding bones and pencil-thin limbs - which encourage eating disorders such as anorexia, a study warns.
Researchers from University of Exeter in the UK show that among thousands of account holders on social media, the images of protruding bones are almost exclusively posted by young women.
This so-called "bonespiration" content features selfies by young women of their skeletal bodies featuring protruding collar bones, hip bones and spines in a variety of poses, researchers said.
Its purpose is to boast about a skeletal appearance and inspire others to achieve the same emaciated look, they said.
The study, published in the Journal of Eating Disorders, analysed 730 images posted.
It found 26 per cent of images showed hip bones, 23 per cent showed jutting ribs, and 22 per cent showed protruding collarbones. Six per cent of photographs analysed depicted the spine.
Academics fear that social media has replaced the pro- anorexia websites, and are becoming an easy to access way of encouraging eating disorders, such as anorexia, researchers said.
They fear efforts to ban the images would lead to others with different hashtags appearing within days.
The study also found more mainstream twitter accounts such as 'thinspiration', which features photos of thin celebrities, also depict young people with protruding ribs collar bones and spines.
Researchers also noted a small sub group of people posting skeletal images with the hashtag 'fitspiration', which is dedicated to inspiring supposedly healthy bodies.
The sites are being used to promote extremely unhealthy body types and are increasing pressure on teenage girls to try to become extremely thin, while contributing to a distorted view of their own body, researchers said.
"Anorexia and extreme weight loss is a serious social and medical problem," said Catherine Talbot, a psychologist at the University of Exeter.
"To tackle this social contagion we need to be aware of the social media platforms being used by young people - mainly girls and young women - which is encouraging extreme weight loss. This behaviour could seriously damage their psychological and physical health," she said.
"Teenagers need to be taught about positive body image in schools and we need to build resilience," Talbot added.