The Ohio report shows that students who used Facebook had a “significantly” lower grade point average - the marking system used in US universities - than those who did not use the site.
“It is the equivalent of the difference between getting an A and a B,” said Karpinski, who will present her findings this week to the annual meeting of the American Educational Research Association.
She has not yet analysed whether a student’s grades continue to deteriorate the longer he or she spends on Facebook.
Some UK students have already spotted the potential danger. Daisy Jones, 21, an undergraduate in her final year at Loughborough University, realised the time she was spending on Facebook was threatening her grades - prompting her to deactivate her account.
“I was in the library trying to write a 2,000-word essay when I realised my Facebook habit had got out of hand,” she said.
“I couldn’t resist going online. You do that, then someone’s photo catches your eye. Before you know it, a couple of minutes has turned into a couple of hours and you haven’t written a thing.” Jones is among the few to have recognised the risks. According to Karpinski’s research, 79% of Facebook-using students believed the time they spent on the site had no impact on their work.
Facebook said: “There is also academic research that shows the benefits of services like Facebook. It’s in the hands of students, in consultation with their parents, to decide how to spend their time.”