Well, now there's scientific evidence to back you up. Research carried out by psychologists in the Netherlands has found that students who use Facebook while they're studying get significantly lower exam grades.
The survey looked at over 200 students and found that among Facebook users, the typical grade average was up to 20 per cent lower.
Study author Professor Paul Kirschner said: ' 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.
'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.'
Students who did not use the site said they devoted more time to studying, spending an average of 88 per cent longer working outside class.
Professor Kirshner, who said he believed that he expected to see the same results in young school pupils, said: 'We should resist the fashionable views of educational gurus that children can multi-task, and that we should adapt our education systems accordingly to keep up with the times.'
Do social networking sites damage children's ability to learn? We want to hear your views.