Reddit has an absolutely massive wealth of community knowledge. If you want to find a community for $thing or gain obscure knowledge on $thing, that’s where you go (assuming there isn’t an old forum post from before Reddit killed forums).

Twitter is where a lot of people still are. If you’re the kind of person to care what a particular person says, that’s where you probably want to be.

Instagram is used by young people who have friends on Instagram.

It isn’t a great system, but it is the system that we have today. This is why legislation compelling Meta/Twitter/whothefuckever to act in an ethical manner is important. Social media is to some extent a natural oligopoly, and unless we get extremely, extremely lucky, the fediverse will always be a niche community.