I am a rather introverted student interviewer, so take heart. 🙂 I don't "hang out" with tons of people in my class, either, and I don't value that highly. However, I DO think it is important that you can carry on an articulate, polite conversation, because this is a skill that is integral to clinical medicine. As an interviewer, I don't want you to be a clone of me, but I want you to demonstrate your individuality--what interesting things have you done in your life? Do you work well with all types of personalities? Do you work hard? etc....
The "fun" question IS an important one. Having reviewed lots of applicant files at this point, I can tell you that almost everyone looks alike on paper. It is the personal particulars that make you stand out. The least pleasant students in my class are those that don't have a life outside of medical school...they are often the most unhappy, as well. But "fun" doesn't have to be partying--it can be reading, running, painting, windsurfing in Nigeria--anything that you enjoy.
If your interviewers, ahem, are really into the club scene, you CAN be interested in what they have to say (this IS a skill you need in medicine...find a way to be interested, even if it's not your cup of tea) without lying about clubbing every weekend. And for medical school, get used to learning to reach a modicum of interest in anything and everything...curiousity is a value around here! 🙂
Good luck!