Then social media happened

Yeah, definitely part of the story. Another thing that happens to all user generated content sites is the following:

• they start small and grow organically, attracting like minds
• they begin to grow faster and they become hard to maintain as a side hobby, including server costs
• they reach a size where server costs are beyond what anyone can afford as a hobby and at least one person needs to make the place their full time job
• ads are introduced because you can turn them on and get money - maybe they’re only show to logged out users or something to control the blowback
• ads on UGC don’t pay a lot so you need huge traffic to pay any actual salaries with them - this means SEO growth
• search engines now shower the site with traffic because it has a deep well of excellent content from its early days, and this is welcome because it drives the ad revenue
• costs also rise because the site’s software was never built for this scale and it needs professional attention and / or enterprise grade service. No one has the know how for the most meaningful performance optimizations or an appropriate caching layer - though many half assed tinkerers will fiddle around thinking they know more than they do
• the shower of SEO inbound blows away any concept of organic growth, which is what made the place to begin with. Now you’ve got plain old anybodies joining and probably expecting instant gratification when they ask a question. Just as the operators are straining to grow the site to the next level, it rots out from under them
• the true blue mods from the old days burn out on this and need to be replaced by rules-based systems and automation
• that’s nowhere near as good and starts to erode the experience
• heroic content creating users are now trapped between the unwashed hordes of the general public and shitty moderators, so they burn out too
• everyone wonders gee what happened to this place and they come up with highly specific explanations, but this regression is nigh universal and you might say inevitable from the start. The only communities that avoid this fate are the ones that close membership and dole out new accounts incredibly sparingly by hand to select individuals. But this works against exponential growth and feels “elitist” and the bills may go unpaid