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I'm sorry, but just a million times no. If Mastodon isn't "a messaging service that intends to connect people across the world", then I can't even imagine what it's supposed to be. Because that is exactly what it is. And if it somehow hit a limit at 10 million total users, then it would be a complete failure. And to say that "it wouldn't really detract from the Fediverse" if you couldn't follow popular people because…
I think you're missing the point of a social network. Barack Obama is not social on Twitter. You can't talk to him. You can't have a conversation. Look at his Twitter feed: when's the last time he's replied to anyone? He issues mini press releases, that's all. You can't email Barack Obama either, by the way. I mean, you can try, but good luck with that. You talk about a messaging service, but it's entirely one-way me…
There are tons of people on Twitter with over 10 million followers who interact with Twitter a ton. They're very "social".
If a social network can't handle people who are popular, then that's a real problem. And remember, people's accounts can start unpopular, and then they gain millions of followers over years. So what do you do, boot them off for the sin of saying things lots of people want to listen to and interact with?