In experiments with a group of female college students, those who logged into their Facebook accounts were more likely to worry about their weight and body shape afterward, compared to women who read online articles about a neutral topic.
For those working on treatment and prevention of eating disorders, the effects of spending time on Facebook may be a factor to take into account, the study team suggests.
"We've done research on peer influences in other contexts - this is the first time we've looked at social media use," Pamela Keel told Reuters Health.
Keel is a researcher and director of clinical training at the Florida State University Department of Psychology in Tallahassee. She oversaw the study that was led by student Annalise Mabe.
The idea for the new study started when Mabe wondered if Facebook use could be related to how women feel about their weight, their body shape and their eating, Keel said.
Some previous research had indicated links between Facebook use and disordered eating, Keel said, but "nothing had really examined whether that meant that Facebook could in some way be contributing to eating disorder risk, so we decided to look at it."
In the first part of the study, 960 students answered survey questions on their attitudes about dieting and their eating behaviors. They were also asked about the amount of time they spent on Facebook.
About 96 percent of the women used Facebook, and on average, they spent a total of about two hours a week on the site.
The young women whose questionnaire answers indicated disordered eating attitudes and behaviors were somewhat more likely to spend more time on Facebook, the researchers found.
"When you find that kind of association, you can't be sure whether somehow the time spent on Facebook is an eating disorder risk or it could also be that women who have higher concerns of weight and shape and are more concerned about their eating are more drawn to spending time on Facebook," Keel said.
"So the second study was really what makes an important contribution in this area because it represented an experiment in which we could demonstrate whether or not Facebook use had any causal effect on eating disorder risk factors," she added.
The researchers identified 84 women from the earlier survey who used Facebook more than two hours a week and randomly assigned them to one of two groups.
In one group, each of the participants was instructed to log into her Facebook account and spend 20 minutes on the site doing whatever she normally would. Members of the comparison group spent 20 minutes on the Web reading Wikipedia articles and watching YouTube videos about ocelots.
After the online sessions, all the participants were asked to answer the questionnaires again, along with questions about their Facebook use and surveys about their attitudes toward eating and dieting.
The researchers found that women who had stronger attitudes of disordered eating tended to place greater importance on receiving comments and likes' on their Facebook status and photos.
Those same participants were more likely to untag photos of themselves and to compare photos of themselves to pictures of their female friends more often.
After 20 minutes online, both groups of students were less preoccupied with weight and body shape, but the women in the comparison group showed a greater decline in that preoccupation than the women who spent their time on Facebook, the researchers report in the International Journal of Eating Disorders.
"We did see significant differences in how their disordered eating risk factors changed after Internet use and that provides a very strong piece of evidence that Facebook use does influence eating disorder risk factors," Keel said.
The study has limitations, including the fact that it didn't look at men, and it's not clear that the decrease in weight and shape concerns would happen outside of a laboratory setting. The researchers also can't say whether other social media sites would have a similar influence.
"This is a very interesting study in part because it brought in the new element of how social media use can actually impact young women's perception of their health, specifically in the context of body image," Yvonne Chen told Reuters Health.
Chen, an assistant professor of strategic communication at the William Allen White School of Journalism and Mass Communication at the University of Kansas, studies how mass media affects peoples' decision-making processes, perceptions, attitudes and behavior.
"The authors in the article talked about how Facebook actually is a risk factor that impacts eating disorders and then in the conclusion they were quite optimistic that perhaps Facebook could also be used to promote health," said Chen, who was not involved in the study.
The fact that "Being around peers who are concerned about dieting and weight increases your own concerns about dieting and weight, also means that being surrounded by peers who have lower concerns about dieting and weight and have a greater body acceptance actually protects you from those problems," Keel noted.
"So I think there's an opportunity to use peer influences in positive ways that are supportive for young women," she said.