1) companies had a desire to grow everyone's networks. Early numbers said, people more connected have more engagement, and the larger a user's graph is, the better we can advertise to that user.
But at some point it also changes what you want to post, or if you want to post at all. Do I want my mom to see this joke? That party photo? What about that guy I met on exchange 10 years ago?
I have 1000 connections on Facebook, and all _their_ connections can be exposed to my post. It might as well be public.
2) Professionals are better at posting than amateurs. It turns out that we rather watch a funny update from a comedian we don't know, than an update from a friend trying to be funny. Social networks are not social anymore. They are mostly TV, but instead of channel surfing, you are doom-scrolling videos from professional creators, not updates from friends.