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.
I'm somewhat optimistic that Facebook will fix this someday, and I didn't want to end on a recruitment blacklist or something in that case. Still, you're right, next time I get one I'll make it clear that this is the biggest reason I'm uninterested, and that if they fix it they should feel free to contact me again :)