Ehhh, at their scale, paying humans to moderate is a tad infeasible, and ai-based moderation is very much a work in progress. For that matter, I think a lot of people would be really uncomfortable if a random human was reading the messages they sent to their mother. They could definitely do better, particularly when it comes to really widely spread stuff, but I doubt that they'll ever be able to get all objectionable content off of the site.