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: some person starts a web forum or creates a cool web app
• it grows organically for a time, attracting like minds one at a time
• everything is awesome and the site’s growth picks up speed
• the site becomes harder to maintain. Software development, security, moderation, and server costs all start to get beyond what the site founders can handle as a hobby.
• 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 shown to logged out users or something to control the inevitable blowback
• people will complain regardless but of course won’t donate a cent of their own to help
• 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 as traffic rises 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 cool 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 site operators are under stress. They may have lapses where they disappear for a while
• someone starts an alternative site promising a return to the glory days of like minds gathering organically. At some point major downtime happens on the original site and drives migration to the alternative(s)
• back on the original site, the true blue mods from the old days burn out on all this and leave - probably one of them is the guy who started the alternative site. Dedicated mods with intimate and deep knowledge and judgment start to get replaced by volunteers hamfistedly applying rules-based systems and automation. That’s of course nowhere near as good and starts to erode the users experience
• heroic content creating users are now trapped between the unwashed hordes of the general public and shitty moderators, so they burn out too. The last-douchebag-standing effect takes hold, whereby the whole place is increasingly dominated by the few grittiest nutjob superusers who hold out / hang on the longest and begin acting like they own the place
• everyone wonders gee what happened to this place and they come up with highly specific explanations, but details really don’t matter - this entire progression is nigh universal and you might say inevitable from the start. The only communities that avoid this fate are the ones that have an extraordinarily easy path to monetization which they use very wisely, or the ones that close membership and dole out new accounts incredibly sparingly to handpicked individuals. But this exclusivity works against growth and feels “elitist” and the bills may still go unpaid as necrosis inevitably sets in without more robust growth. One day the site suddenly goes offline.