I still host one of those 20+ year old forums. The Fediverse is different. With forums (and HN/Reddit) you immediately had good sense of them being for you or not. With the Fediverse to have to commit to servers and even then you don't know if they are right for you unless you try and spend time customizing your feeds/follows. It's a lot of work and you don't know if it will pay off. I tried again today and so much of it has no focus at all. It reminds me of this exchange from the Good Place TV show:
Chidi Anagonye: So, making decisions isn't necessarily my strong suit.
Michael: I know that, buddy. You-you once had a panic attack at a make-your-own sundae bar.
Chidi Anagonye: There were too many toppings, and very early in the process, you had to commit to a chocolate palette or a fruit palette. And if you couldn't decide, you wound up with kiwi-Junior Mint-raisin, and it just ruins everyone's night.
How did you get that 'good sense' with forums back in the day? I did that by reading into the community. Just look around on the server, see the posts on there, and see if there's a connection. You can do that with Mastodon servers too. And it's not that high stakes. You are not stuck on that server, if you find out that it's not quite the right fit, you can move to another server. In fact, if I were to look at my following list, I think I see more people from outside the mastodon server I joined than people from the same server.
For forums you can look at them for a minute or less and figure it out. It's same with HN and sub-reddits. They are information dense and don't require signing up or customization to figure out if you can curate them into something you might like.
Compare that to mastodon, step 1 is pick a server https://joinmastodon.org/servers Very few are actually topic focused. Even the ones with themes have a lot of deviation. Even picking the popular safe choice - https://mastodon.social/ , I can't tell if I would ever like it. I don't like what I normally see but considering I can see 2 posts at a time and a significate portion are animals or other topics I wouldn't visit a forum about, it doesn't feel like I would.
And the animal thing is common across most of the servers. I understand it, I have dogs. But it's a side effect of the medium not having a coherent focus. It feels like I'd have to spend so much time to turn it into something that I'd like that I'd be better off staying with forums.
Mastodon seem great if you want to follow people and be social which is kind of the point of social media. I want to follow areas of interest, not people.