Faceboook Groups and Discord are very useful for learning a variety of things.
But the discovery of such private groups is not happening based on the content - like you might find a forum on the open web because of a question that has been indexed by Search Engines.
Also, the search within these apps is pathetic.
The content seems very ephemeral, I can't find really old posts unless I put in a lot of efforts.
Reddit has been good in this regard, especially when you use Google for searching posts.
I hope they don't screw their user experience further to prevent AI companies from getting their data.
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To answer your question, I believe the open web will survive.
I hope the more personal, less commercial ( SEO optimized ) content might rise to the top if the commercial outlets block access to content.
More likely we will have AI feeding on AI generated content that will be crawled by Google AI and recommended to us by AI.