Facebook revolves around user interaction, that is videos, photos and text (in various forms, even links to external sources).
Adult entertainment deals with those too, but those sites are mostly used like YouTube with lower session times.
Every minute "510,000 comments are posted, 293,000 statuses are updated, and 136,000 photos are uploaded" on facebook, on Pornhub it is 22 video comments and 122 messages. Xhamster should be comparable to that. The difference in volume is striking.
Now with lower volume it is easier to moderate. Scanning videos isn't easy but AI should help, but AI these days fails for text, at least for anything else than an easy classification task. Hate speech isn't just random insults, there is more to it, and current AI systems can't possibly understand the whole context to something.
Even if content wasn't released immediately on facebook, there aren't enough people in the world to moderate the amount of content posted on facebook everyday.
I really think the comparison of those sites doesn't make sense.