It doesn't matter that most writers have a negligible impact on society. You write as if writing to people, whether individual people or to a large group of people. You are inevitably going to affect some people, whether a small audience or a big one, and your effect on your readers should, I would think, be the same as the effect you want to have on any people you interact with?
I did also mention that I believe we have a duty to God / DoNoHarm (i.e., don't write KKK propaganda), then to family, then to society, then - if the others are handled - to work (or in this case the art). But I don't believe we need to push obligations onto people who aren't ready to meet them. Most people here, for instance, are still some of the basics about the art - it seems presumptuous to tell us we should be improving the art. Most of us lead limited lives and have a small audience - it seems presumptuous, to me, to tell us we need to worry about bettering society when we barely have readers.
Yes, maybe we'll get to a point where those things become real to us. But let's not get ahead of ourselves. In terms of the moral duty of our writing, it really is negligible right now. That's just the reality. We're screaming into the void of the internet, and maybe a few people are listening, but even if they are, they're not listening closely enough for it to make a difference to them. Not yet at least.