Consider that Facebook has entire teams of people dedicated to figuring out what behavior is compatible with their business model, and they also have more direct data about where revenue comes from than any of us here do (at least those of us not bound by a Facebook NDA!). There's probably a PM whose entire job is just putting together these reports and they're getting a Facebook salary for doing so. It's definitely possible they're all misguided, but they're a very successful company and they've probably considered the tradeoffs of investing in these efforts versus not.
Also, consider that e.g. 4chan is making nowhere near as much revenue as Facebook, despite also providing a forum for people to post things and selling ads next to those posts.