I'm not sure how things like smoking rates reflect narcissism. Ditto for taking calculus.
The point is that there is no evidence for an increase in "narcissism" (however that's defined), selfishness, irresponsibility, entitlement, immediate gratification, unwillingness to work for things, poor decision-making, or any of the other various ways that the supposed personality flaw of Gen Y gets described. No matter what measure you use, Gen Y seem to be behaving like pretty responsible young adults.
Plus, how much of that civic involvement is in aid of college application fodder? And how much is required by schools?
Two-thirds of high school students who do volunteer work continue to do so after high school. There is surely some natural attrition even among those who volunteer for disinterested reasons, so the answer to your question is "something quite a bit less than 1/3rd."