I think a lot of the smaller social media sites started getting exploited with spam and various other problems and really couldn't keep up technology wise, make them safe anymore, or make any money. So rather than having countless smaller sites all catering for various little niches we've ended up with a handful of really big generic corporate sites...
That's not to say there isn't countless other reasons they don't appear to be as full of life as they once were but sites like Facebook, Twitter, Tic-Tok to name a few they probably mop up bigger user numbers now than they ever have and people generally just flip flop between them kinda sites so you don't really get community based social media sites anymore.
I mean it's a big question as how people even use social media has changed so much over the last decade. Like we've moved from login in with PCs and laptops typing 80 words a minute to using tiny smartphones where people login type a couple of brief comments and log back out again in between doing other things, so they're not always as engaged as they were when it was all new and exciting.
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