As others (and OP) have noted, Twitter has a lot of different facets to it, and the right tool to replace it will probably look different for each of those facets. And with so many facets, I think it’s going to be impossible to summarize all of the options here, but I’ll share a couple of thoughts.
I was an early adopter on Twitter, creating my account in 2007, but took several intentional breaks over the years, and haven’t posted on it since early 2021. It was at its best, for me, when it was a vehicle for “ambient intimacy” — being aware of the various things that my friends* (some online, some real-world; some were one-sided relationships, some mutual) were doing. I wasn’t following too many people, and the chronological timeline meant that I could dip in, get caught up, then go about my day. That usecase maps pretty well to Mastodon, and I was able to get an account on a server with others with similar “early days of the web maker vibes”. I think getting on one of the large Mastodon servers would be significantly less enjoyable.
For those new to Mastodon, a good metaphor is that it’s like a bunch of backyard parties, with everyone at the party slightly aware of some of the other backyard parties. Occasionally, folks in your backyard party will share something they’ve overheard from one of the other parties. So if you’re in a massive party, with lots of people sharing stuff you don’t care a ton about, it might not be that valuable. On the flip side, if your party is too small, there might not be enough interesting stuff getting shared to make it valuable. So there’s a balance. I was lucky to find a backyard party that worked well for the facet of Twitter I valued. (You can see a not-comprehensive list of Mastodon servers (the backyard parties) here, but I’d be wary of any of the ones where membership is just unmoderated.)
Unfortunately, one of the amazing things about Twitter was how messages and ideas could disperse widely, and I think it’ll be some time before we see whether something similar evolves at Mastodon. For example, I learned so much about the lives and experiences of Black people via Twitter, and it had a profound impact on how I see the world. I’m not sure that those messages will be amplified as easily on a platform like Mastodon. Another facet … a friend mentioned how Twitter had utility as a place for collective grieving, when appropriate. I’m not sure that Mastodon will offer a similar experience. So it’s not a 1:1 replacement for Twitter, but I’m enjoying it so far, and it does feel like the early days of Twitter in that regard.
A friend has a small business, and her usecase for Twitter looked very different from mine. She used it to share news about new products, or deals, or relevant news to her small business, for people interested in that. If I were her, I’d be working to develop an email newsletter — something I controlled, where I had the email addresses of my audience and could reach out with my own messaging. I’d consider doing a monthly (or whatever cadence) newsletter, to keep my business in peoples’ minds, and to share those same bits of relevant news. I’d also create a blog where I could capture that same content, or write even more frequently. And, yes, having a signup for the newsletter on the blog, so more people could opt-in to the mailing list.
Similarly, if I were a journalist, or other person who made a name professionally by analyzing things, I think I’d be focused on trying to establish some means of communicating directly with the people who want to hear from me. Kevin Kelly wrote about the idea of “1,000 True Fans” … I’d be thinking about how I could connect with those people.
I guess the summary take is this: Relying on tools made by corporations like Twitter, Instagram, and Facebook makes sense at first, because the posting experience is easy and the network is part of the value they provide. Over time, corporations’ incentives are going to diverge from most peoples’ incentives. The more that individuals can do, then, to own their own content and means of distribution, the less beholden they’ll be to platforms and the decisions those platforms make. I recognize, though, that people are busy, and nobody wants to have to maintain their own server if they can avoid it (and many people aren’t able to). So it’s tricky.
I don’t think there’ll be a true replacement for Twitter, but I think as people consider the facets of it they got value from, they’ll be able to cobble together approximations. I suspect the primary value people got from it, though, was “the network”, and it’s going to be hard to figure out how to reconstitute that. And I think it’s good that you’re thinking proactively; it’s sort of like preparing for an earthquake or tornado … you can’t know for sure what services will go down, but it’ll be a lot easier if you’ve put in thought and prepared ahead of time.