If there is a conversation then I reply.

If there is a good comment or some thoughtful input I zap.

If the comment is contrary or even negative but still the person did the work to write I zap and comment.

If it's just ignorant drivel I comment but I won't zap. Sometimes I've gone back when I was angry at someone and zapped them anyway because I was affected.

Sometimes I've down zapped.

Mostly I try to seed the conversation if it makes me aware, think or educate.