You can handle it several ways:
1) Cut them off - "We don't discuss parties during recruitment because we want people to get to know us for stuff other than our fantastic social occasions"
2) Turn around - "Sure. we have great parties, and we also have service day, Must See TV Nights, etc..."
3) Upfront - "I don't drink because I'm always the Sober Sister or photographer catching my friends (note: friends, not sister) at their silliest"
4) Misdirection - "Where did you hear that?" if they say "My friend/parents/hallmates" then ask about that person, if they're Greek, where they're from, etc. Can lead to a whole other conversation
As far as drinking being equated with your commitment to the org., that's just wrong and unfortunate that someone would think that. I would just emphasize your commitment in the number of offices you've held and your social contributions outside of the bottle.
