A study has found that the exam results of those who used the social networking site while working, even if it was on in the background, were 20 per cent lower than non-users.
According to researchers, the findings put a dent in the theory that young people's brains are better at multitasking on digital gadgets.
"The problem is that most people have Facebook or other social networking sites, their emails and maybe instant messaging constantly running in the background while they are carrying out other tasks," the Daily Mail quoted study author Professor Paul Kirschner as saying.
"Our study, and other previous work, suggests that while people may think constant task-switching allows them to get more done in less time, the reality is it extends the amount of time needed to carry out tasks and leads to more mistakes," he added.
His team studied 219 students aged between 19 and 54 at an American university.
It was observed that the Facebook users had a typical grade point average - a score from zero up to four - of 3.06. Non-users had an average GPA of 3.82.
Those who did not use the site also said they devoted more time to studying, spending an average of 88 per cent longer working outside class.
Three fourth of the Facebook users said they didn't believe spending time on the site affected their academic performance.
The study by Open University in the Netherlands will be published in the journal Computers in Human Behaviour.