My feeling/understanding: Because of the culture wars and privacy concerns, many regular people who used to be quite active on FB stopped posting things altogether on Facebook. The risk outweighed the benefit. So most people were left with a handful of people in their network who would crank out generally quite uninteresting posts.

This combined with FB's agenda to monetize the service... here we are.

I now only use FB to check in to a few very local groups (population of 7k) once in a while. The signal-to-noise ratio is really bad, even there.