They're very close to that. And the same is true to other content, e.g. Spotify, Amazon Kiddle, Netflix for films, YouTube for video, etc.
People still go to other sources, but increasingly less so, and those sources all increasingly fringe (Daring Fireball? Compared to Facebook and the like?). Nothing like the wild west (and much freer and distributed) market it was 10 and 20 years ago.
Between the 3-4 big social aggregators for example, there goes 80% of the internet advertising pie.
Gates (as quoted below): "There will be no content club in my life time with $4 billion revenue...you are on drugs if you think that".
Meanwhile there are like 5 "content clubs" with much higher revenue...
[1] https://qz.com/333313/milliions-of-facebook-users-have-no-id...