not drink for health reasons
Yes this sounds plausible. Even if the person isn't unhealthy maybe their family history is such that they still don't drink.
have a less active social life etc
As a semi-introvert[1] I always wonder how true this is for the minority of us that much prefer this so called social isolation. I'd take a night alone or with a small group of friends over a bar or conference absolutely every time always. Of course that is assuming there are no other external influences - which is rarely (I'd say never but we're on HN...) the case. All that really matters is whether you are suffering from anxiety rather than loneliness[2].
[1] I'm XNTP somewhere between INTP and ENTP on the meyer brigs http://en.wikipedia.org/wiki/Myers-Briggs_Type_Indicator
[2] I can't remember the last time I was lonely or bored. Hmmm, is this really that unusual amongst similar introverts? I just consider myself somewhat curious and self-motivated.