The incentive in the GMail case is still there. If you think your action makes a friend better off, then the action is valuable to you. Whether this act stems f

The incentive in the GMail case is still there. If you think your action makes a friend better off, then the action is valuable to you. Whether this act stems from intrinsic selfishness or benevolence, is left for another time :-) Anyway, I doubt anybody really thinks about this before coaxing their friends, but broadly speaking "so do I" holds true.