I spent most of last Friday keeping an eye on 4chan's /pol/ after finding out that morning that users were planning an attack on my job.
Even just looking for one day, it took a serious emotional toll on me. I've definitely seen some awful things on the Internet but the constant bombardment of hate speech, racism, anti-Semitism, and all sorts of disturbing images and text over the course of 6 hours made me feel physically sick a number of times, and I had to take extra care to rest the next day.
This is anecdotal of course, but I can't imagine what the Facebook moderators go through having to process at least one ticket a minute.