Facebook does provide tools to segment one's friends into groups to whom content can be privately posted, but from a UX perspective they're deemphasized. I see a lot more social conversations moving to group chats (some on Facebook Messenger, some on GroupMe/WhatsApp and the like), because membership is determinate, you make the choice of target audience before being prompted to make a post, and the context into which you post is not a newsfeed (which we've been trained to think is global to an entire circle of friends) but just that specific chat history. (I'm sure there's some psychology/UX research in that people don't intuit that a "posting destination" can be distinct from the list of content you're viewing.)
Chats are just a "safer space" to be oneself. And if Facebook wants to capture one's true self and preferences, it will need to evolve beyond the idea that a single newsfeed is the place people want to do that.