"I don't do mainstream social media. Can rumours be traced back on it? What are one's chances of even getting copies of the damage? Does truth have a chance, or is it mostly mob rule?"
In the legal system it might, but in terms of slander, a lot of times when the damage is done, then that's it; you have to bite the bullet, and that is not easy. This said, if something is written on the internet about you that is not true, there may be a way to get it removed, but if a person has done business with you and had a bad experience, then I think they have a legal right to write about the specifics, such as on Yelp, but you would have to research that. Am not sure. Once you have a bad review on Yelp, which I suppose anyone could make up, then that could affect your business and would be hard to trace.
I do not know what you think of the concept of karma, which does not really mean that any human being deserves bad treatment, but can be a helpful way to put difficult and sometimes very painful experience into a more functional context where a person might have a better chance of thinking in a way that would help work this kind of very difficult experience through.
I experienced an instance of outright slander many years ago that practically destroyed my life. I tried to process it from the perspective of karma, and that was very helpful to me. To anyone who is experiencing a lot of this, not to imply you are at fault or that to slander is in any way excusable, but it is always a good idea to look at ones own behavior with an impartial eye, kind of fine comb through it to see if anything you are doing could in some way be eliciting certain kinds of negative responses.
To the person who wrote that they get very upset when people say repeat unconfirmed malicious stuff about other people in his presence. Yes, to say that kind of stuff is a form of what some call "wrong speech." Do not participate in it and from your example others will learn. Actally, though, you are kind of unique, as most people do not mind so much when someone says something false about someone else, but when it happens to themselves that is a different story.