My main Forum is alive and well, because people will still Google (or quack) a topic and be presented with search results that include my niche community near the top.
Facebook can’t do that nor prevent my site appearing.
Facebook has tried and failed to replace the open internet along with Apple and its obsession with native ‘apps’ (which are also dying, paid apps almost completely dead).
It is not the future. Facebook is a passing fashion. Facebook is an enormously expensive facade. Their costs are only going to rise if they are forced to more tightly police content and take full responsibility for it. And in doing so they will also alienate users (and doing a pretty good job at that already!)
The cloud revolution means my costs are tiny!
The open internet will prevail because most technologists don’t work for Facebook and want their products to be visible on search engines.
Reddit is probably a bigger rival but that fight is on more even terms.
I realise the “open internet” is a dream in some countries but it is something many countries will seek to defend whilst others will sadly seek to control it. 
We must do everything we can to resist these monopolies!