Facebook certainly has the potential to impact a student's study habits, but so do X-box, Play Station, computer games, smart phones, sports, friends, etc. In my day, well before Facebook and other social media were around, we used a different form of technolgy to 'waste time'. It was called the telephone. I wish I had a few of the hours I spent on the phone while in my teens back now!
The bigger issue is one of priorities and student's being able to set them. There will always be some new, shinny thing out there to distract us from the task at hand. And honestly, some level of distraction is good. Remember, all work and no play makes Jack a dull boy. However, all things in moderation is a good rule to follow.
As much as some of my generation lament how much time and energy students spend today on sites like Facebook, it's not too hard to remember my parents having a very similar talk with me when I was a teenager about how much time and energy I was wasting on the telephone.