It's probably a bit controversial to say this, but haven't large sites had to deal with these problems for years by the time the likes of Facebook/Twitter/YouTube/Instagram/whatever came around? It's obviously horrible doing this as a job and it's obviously a bit more common on Facebook than say, a large old school internet forum, but... at least in this case people are paid to do this. Reddit mods and old school for…
I also think part of the objection here is around off-shoring. It's similar to other complaints about off-shored jobs: we're farming out unpleasant work to people that get paid less and have much less support (e.g. mental health).