Facebook users have lower overall grades than non-users, according to a survey of college students who also ironically said the social networking site does not interfere with studying.

That disconnect between perception and reality does not necessarily mean that Facebook leads to less studying and worse grades -- the grades association could be caused by something else. However, it does raise more questions about how students spend their time outside class on activities such as Facebook, part-time jobs and extracurricular activities.

"I'm just saying that there's some kind of relationship there, and there's many third variables that need to be studied," said Aryn Karpinski, an education researcher at Ohio State University.

Her study found that Facebook user GPAs were in the 3.0 to 3.5 range on average, compared to 3.5 to 4.0 for non-users. Facebook users also studied anywhere from one to five hours per week, compared to non-users who studied 11 to 15 or more hours per week.

However, Karpinski emphasized that correlation does not equal causation, meaning Facebook use might not be the culprit behind lower GPAs or less study time.

For instance, students who spend more time enjoying themselves rather than studying might tend to latch onto the nearest distraction, such as Facebook. Or students who use the social networking site might also spend more time on other non-studying activities such as sports or music.

The study did show that students who work more hours at jobs spend less time on Facebook, while students involved in more extracurricular activities were also more likely to use Facebook.

Such results may help Karpinski and other researchers better understand the profile of the average user on the popular social networking site. Early findings show that over 85 percent of undergrads use Facebook, compared to 52 percent of grad students.

Karpinski herself does not use Facebook, although her coauthor does. She expressed some bemusement over how quickly students rose to the defense of their online activities, even though she took care not to introduce a positive or negative bias into her survey questions.

"They're very territorial about their Facebook," Karpinski told LiveScience.

The research will be presented at the annual meeting of the American Education Research Association on April 16.