You forgot one major thing. The algorithm. Facebook decides what you see and what is being recommended to you. If they recommend to you something that has more engagement but is fundamentally false, we have a problem. It's one thing when it's just a pure communication platform (whatsapp), another when you push/promote some content in the feed over something else. No they don't have a master evil plan, but they are careless and don't care about their own responsability. One example where it becomes a problem: it was proven that there was violence in Myanmar triggered by false information that was pushed by the algorithm to more people