Young women were more likely to compare their own body with that of their friends if they spent more time on social media, and Facebook in particular.
Scientists found that more time on Facebook could lead to more negative feelings and more comparisons to the bodies of friends.
Researchers from the University of Strathclyde, UK, Ohio University and University of Iowa in US, surveyed 881 college women about their Facebook use, eating and exercise habits, and body image.
They were able to predict how often women felt negatively about their own bodies after looking at someone else's photos or posts, and how often women compared their own bodies to those of their friends.
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They also found that for women who want to lose weight, more time on Facebook led to more attention being paid to physical appearance. This included attention to one's body and clothing.
Previous studies have examined college or adolescent girls and the effect of Facebook on users' body image over non-users'.
However, this is the first study to link time spent on Facebook to poor body image.
"Public health professionals who work in the area of eating disorders and their prevention now have clear evidence of how social media relates to college women's body image and eating disorders. While time spent on Facebook had no relation to eating disorders, it did predict worse body image among participants," said researcher Petya Eckler.
The attention to physical attributes may be even more dangerous on social media than on traditional media because participants in social media are people we know.
"These comparisons are much more relevant and hit closer to home. Yet they may be just as unrealistic as the images we see on traditional media," said Eckler.
The findings will be presented at the Annual Conference of the International Communication Association in Seattle.