Earlier quoted context omitted.
> This isn't about "celebrities" (a word I never used), it's about popular people all around, like authors, journalists, and so on. I don't want to quibble over terms. Celebrities, popular people, whatever you want to call them. > And popular figures absolutely interact with and reply to tons of their followers quite frequently, that's the whole point. And popular people have plenty of places where they can do that.…
> And popular people have plenty of places where they can do that. But why does everyplace have to be amenable to people with 10+ million followers? This is where you're wrong. Twitter is the only place they can do that right now, where the audience is for that type of content. And people are positioning Mastodon as a better alternative to Twitter. So Mastodon needs to be amenable to people with 10+ million followers…
You asked about email, I'll counter with actual message services. For reasons clear only to the people involved, I have contacts I can only reach via Signal, or via WhatsApp, or via Telegram, or via WeChat, or via Discord, or via Slack, and so on. How do I cope with this? I just do. Similarly, if I had to visit eight different web pages to track my favorite super-popular people, I just would. Or I'd do what I actually do, and follow almost none of them, instead letting people who are terminally online quote and link to the ones worth knowing about.
If ActivityPub filled up, and couldn't take on any more users, then I would first expect technical solutions like a sharp rise in relay servers, which combine the output of many servers into one outbox to be polled. After that, assuming there was some sort of hard limit that couldn't be overcome that way, I would expect that I would need to create a new account on some other ActivityPub-like network, or even a separate unfederated network also running ActivityPub. My chosen client (Mastonaut) can already support multiple ActivityPub accounts on different servers, so my experience would likely remain the same.
You initially suggested that the Fediverse can't scale. It's worth noting that Twitter itself fell apart long before it reached anything like today's mastodon numbers, and the "Fail Whale" was a constant companion in 2008, when Twitter had only one million users, 200k of which were active per week[0]. So already ActivityPub is scaling better across the network than Twitter itself once did all by itself. Maybe Mastodon (Ruby on Rails) will fade over time, replaced by something written in a more efficient language (like Twitter was). Maybe ActivityPub itself will be converted to use thrift or grpc or similar. Maybe ActivityPub can support 100 million users just fine. It seems to have handled going from 300K monthly active users to 2.5 million monthly active users without trouble[1].
The total Fediverse seems to have more than 8 million total users now. I guess we might find out.
0. https://techcrunch.com/2008/04/29/end-of-speculation-the-rea...
1. https://blog.joinmastodon.org/2022/12/twitter-suspends-masto...