Content warning: discussion of abuse.
I was recently in a discussion of safety, "me too", and responding to harmful behavior where someone wrote, paraphrased:
Discussing things like this, publicly and in writing, is dangerous. Posting negative things about people could get you sued for libel, or even physically assaulted if the person takes it poorly enough. Why would you put yourself at risk, just to get something off your chest?
While they're right that speaking up is risky, the idea that people who make public posts are doing it lightly doesn't match what I've seen. Most people who post know the risk, but decide to go ahead for important reasons including "no one else should have to go through this" and "if people know bad behavior will come back to bite them they'll behave better."
In November 2017 someone in one of my communities posted publicly about someone who had abused them, and then in September 2018 two people in another of my communities posted about a different person.  While these two abusers were different in a series of ways, they had a lot in common: they were relatively popular, they used age, inexperience, and intoxicants to take advantage of people, they had abused a series of people, and there were whispered warnings but not everyone heard or believed them.
In both cases, the public posts completely changed the situation by pulling all of this into common knowledge. The abusers were rightly ejected and communities started working on figuring out how they had let things get to this point.
We need to build a world where this is not needed, and where we have good social systems for handling harmful behavior. I don't know what this should look like yet, however, and all the proposals I've seen have good and bad parts. In my corner of the world I've been working on this with BIDA's safety committee, and I'm glad that communities are generally taking this more seriously. We also need to handle this on a personal level, supporting friends who've been hurt and holding others accountable when they hurt people. But while stopping abusers should not require this level of personal risk, as long as it does I have enormous respect for people who decide to take that risk and speak up.
 I'm not linking the two here, even though both are world-readable, because they've served their purposes and I'm not trying to put more attention on those two cases in particular.