It's a thin line, but mostly I'd regulate the platform to not allow anonymous slander. Between individuals, it's most likely a civil issue, I'm not a huge fan of having criminal law creep into everything.

I'm pretty sure that "no anonymous slander" reduces the amount rapidly, so you end up with a bunch of legitimate accusations that people are willing to stand for because they can provide evidence. The whole "I don't like his hair, so I'm going to make up stories about him" stuff goes away.