If they hear them from someone they're actively attempting to recruit, I expect it would carry more weight. At the very least it will make FBs HR department aware of the issue. As the parent commenter said, there are a lot of people at FB who aren't even aware of these issues.

It's more likely that it will make the candidate look like an asshole.

I don't think so. I regularly response to Amazon recruiters with "your reputation for treatment of your developers is poor, and therefore I'm not interested." It helps provide a data point to drive change, something that someone can point to when arguing with their higher-ups.