Yes, 15000 is a lot. But >2 billion people is a lot more. Wikipedia has far fewer people employed on content moderation yet they seem to fair just fine. This number throwing on Facebook "doing an effort" is an attempt to keep the toxic business model that they have a live at an acceptable rate. How many people would they save for hiring if they'd change the timeline into a chronological one instead of their current one? It'd be much harder to get as much attention than, but also less addictive, which is a net good for every human except for Facebook as a business.