(apologies for the ridiculous length of this, and totally understand if you pass up reading the comment, it's also a bit incoherent/lazy/quickly put together)
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To me, the real problems are the following. All related to "social media" but not social media in itself.
1) The Stream (timeline, whatever you want to call it). Instead of static single "pages" that linked to each other as separate entities with their own internal logic, everything became one flow, and more importantly "impermanent" (e.g. the way we think of "content" is no longer about an archive or repository of things, but rather, moment to moment). A reverse chronological sorting isn't bad in itself. Even the old long form setups like blogging and livejournal etc all supported this method of viewing content - but add in the following issues for a perfect storm.
2) Limited text length (blame Twitter and Facebook, and item 3). By limiting the length of text we create another "short attention span" mindset. Twitter, by it's origin, did have a specific need for short text due to its SMS origin. W/FB - the old 420 char entries was limit to what one can say and encourages short, quick, non-discursive entries. This may have been prudent both to reduce data retention costs and keep people "engaged" (though I don't think the side-effects of this were intentional at first)... This helps to feed the short dopamine burst/feedback. Neither of these were intended for deep discourse.
3) iPhone (and Android; But, frankly it was the iPhone that really got the phone and modern style of apps/ecosystem to where we are now). Having a smaller screen that was "always on" helped to push short text & images instead of longform content, plus the instant feedback loop, wherever you were.
4) "Always On" internet via small devices vs "Log In" standalone bulky systems. Instead of coming home to a computer as a separate space that you had to intentionally log into (in dial up days; or at least turn on in cable/fiber days), we no longer had to set aside a sort of "sacred place". The space no longer being localized, it was everywhere. While this is part of item 3, the form factor of 4 meant that longer form content is still easy to produce whereas with the form factor of 3 means interaction is limited and more difficult to use due to the limitations of onscreen text/keyboards (though advances made it easier, it's still not the same as sitting down and writing a big ass blog post).
4) The death of "blogging"/Google Reader. While blogs exist and come and go in fashion, Google killing reader really was a sort of deathknell. This was more due to the rise of "social media" though, not a precursor. But it helped to cement that long-form content was "dead". Blogs still exist, but aren't near as popular.
5) Images and Embedding/Walled Gardens: The removal of easy interactivity between platforms locking people in to the systems. While in some ways this is a nice convenience (take a URL and past into FB and BAM there's the image all formatted nicely (usually)) - unlike say Livejournal where you still had to type in the img src tags (or use the visual editor that would pop up a window for you to paste the url, that would, after saving/posting the page, would render the image). Sharing memes and other content with a "share" button meant you no longer had to create the content, you could just share what someone else said if you thought it was funny, were angry about it, etc. Meme replication become much less intellectual and lazy. "Engagement" didn't come to mean actual long thoughtful replies, but just an emotive "click" on an emoji to signal ones preferences towards said content. No need to discuss. Sure the option to do so was there, but who has the time when the internet is always on and there's always more content.
6) "The Algorithm" - that which keeps the "engagement" happening and as fast and deep as possible. Just one more click. One more hit of interaction. Keep the people "engaged" as long as possible on a platform.
Of course, long form still exists, and forums like HN allows for a better more thoughtful input on these things. But HN isn't really "social media".
I'd consider blogs and similar forms a proto-social-media (or, with livejournal, honestly I'd consider it the original social media, but without all the algorithmic bullshit that let you organically find people with similar interests).
Finally - I think the drive of FB to have everyone put everyone they know IRL and the ease of "add all your friends from your contacts" makes that also much easier. By adding IRL people, it changes the dynamic. When a lot of us came online, we were looking for an escape FROM the people IRL. We wanted a new space, away from "out there". Eternal September, the normies invaded. They were everywhere, and at all times. And people add them because it's rude not to. And the escape that the online world provided with similar minded people you were hoping to find was reduced back to the friction of "the real world" and the same fights. Note that alone isn't bad in itself. A bubble is also a bad thing.
But further:
7) Ideological Techno-utopianism that doesn't take into account all the corporate power behind the algorithms, the data sharing, and the means of keeping engagement (by rage-baiting, bad-faith-actors, etc...) I used to be one of them, I used to crow about JPB's "Declaration of Cyberspace Independence" in the 90s. Even until maybe 2015 I had hopes. But slowly I began to realize these architectures amplify the worst tendencies of social interaction, purposefully or not. My point with the 90s utopianism is that this Red Book reads like it's 20 years out of date, even by the time it was written, but maybe not. Maybe it just took time for the iPhonization/SocialMedia effects to take root and alter society on the level it has.
"Convenience is the enemy". I don't know the answer, frankly. I know there are a lot of people who try to bring back "the old days" with things like indieweb movement and small shell accounts for kids to try out what the old dial up world was like, and people get interested, but the mass pull of the large places is too great. The power of having an audience (which is the magic of social media - that stream makes it EASY for everyone to just dip their foot in and out whenever they want another nibble from the Dopefish^TM on their feet). There's the tipping point where everyone's friends finally jump ship, this happened with Livejournal and Myspace over to Facebook. Or now Twitter/X to Bluesky. Early adopters watch as the masses come to their little place, sometimes they are happy about it, but other times they see what they thought of as "their" space now invaded by an outside culture. Bluesky probably not so much since it's still quite like Twitter, and most of the people coming in are from Twitter, but for other places there are cultural shifts. Another big change I can think of is how Reddit went from a pretty technical place early on to one full of lazy bro joking when Digg users migrated en masse, and the vibe shifted radically even though Reddit was more bountiful with content.
Some of this will always be a tradeoff (it's called Eternal September for a reason after all), but some of this are design choices. I wish I could say that those designing these things honestly have the best interest of society at heart, but after watching enshittification play out the past decade+ it really feels like good intentions don't mean shit and a lot of hard work needs to go into making a productive place.
It helps when engaged community members are helping run things (whether good reddit mods, or mods on metafilter, or people like dang here on hackernews). Free for all media isn't bad, and I'm not saying every site needs to have mods, etc... But... It does seem like people who have an intentional community and a vibe they are trying to retain can help inculcate a better social atmosphere than shoving everyone into a clowncar walled garden and expecting civil society (especially when algorithms feed on making those dopamine hits for engagement).