Looks like Facebook may be hurting your grades:
Facebook users typically ranged a full grade lower than those of nonusers — 3.0 to 3.5 for users versus 3.5 to 4.0 for their non-networking peers.
From an economic standpoint, it makes sense. Time is limited, if you spend less time studying because you are using Facebook, your grades should go down. I wonder if teachers on Facebook are less effective than those not on Facebook?