Common sense would seem to dictate that students who spend extraordinary amounts of time doing anything but studying would have lower grades. Read this recent study that indicates a connection between uber-hours on Facebook and low grades for college students.
The only issue I have with the study is that it used 219 students from The Ohio State University. Hmmmm, with or without Facebook, the GPA component was already tainted.
"Through the computer, the heralds say, we will make education better, religion better, politics better, our minds better — best of all, ourselves better. This is, of course, nonsense, and only the young or the ignorant or the foolish could believe it. " — Neil Postman, author of Amusing Ourselves to Death: Public Discourse in the Age of Show Business