Don't let anyone kid you. ITs really bad. It won't really sink in until September when you would normally go back.
If you are in grad school that may alleviate some of it, but otherwise its like . . . Withdrawal.
Not just the fact that you might be extraordinarily active in your activities, but just the face recognition.
All of a sudden you go from a high percentage of people knowing you all over campus . . . to relative anonymity.
No matter what else you are doing, working or straight Grad school, its really a pale imitation of the excitment of volunteer activities.
Good Luck