So, what are we writing for? Can writing make a difference? Or is it just entertainment?
Of course there are examples of non-fiction which has helped to shape and alter public opinion, but can fiction?
The danger of setting out to make a difference is you may fall into the trap of preaching which is one of the biggest reader turnoffs. And fiction, like any art form, is only partially in the mind of the author. Every reader will take something different from it, depending on their own attitudes. Sometimes they will even take something that was never intended, had never even occurred to the author. Is that a bad thing? Is there a right and wrong interpretation of a book? I don't think so.
All we can ever do at any given moment is be true to our own vision of the story. After it goes out into the world it's up to the reader to decide what we are trying to say. So our sweet love story may be seen as a scathing indictment of the class system, our edgy thriller a critique of modern day morality. It's in the eye of the beholder and not completely under our control.
So I think fiction can make a difference but usually unintentionally. Readers are too many and varied a group to react identically to the same story. Of course, in times of change a novel can capture the zeitgeist of the moment and become emblematic, for example Harriet Beecher Stowe's Uncle Tom's Cabin. But such instances are rare and unpredictable.