Every instance would only get the messages that its members have subscribed to. How can any system be more efficient than this?

A $5/month VM could ingest millions of messages per day. What's the problem?

Build it and get your friends to join you.

Taylor Swift is the problem. In terms of the system design and architecture, it's an interview question for a distributed systems engineers. You've got a superstar user, with 89 million followers, how do you scale every aspect of your system to handle when she posts? Naturally you're object and say that Taylor Swift isn't going to moving to TekMolTwitter, but pwg said it won't work after a certain size and you said why not, and the short answer is that it doesn't scale past N users, and you can just cheat and say N is higher than you want to care about. We could do a bit of back of the envelope math to see that notifying 15 million users will saturate the gigabit link on you're $5 VPS if each notification packet is 64 bytes, and then design all sorts of queues and caches and redis and and and. It's a fun interview question (and practical problem for Twitter/X) but at the end of the day, if you believe in it, just go build it and get all of your friends and family to join TekMolTwitter (or Mastodon). It's entirely within your capabilities in 2025 to just go out and make something like that, so the thing is, if this is a something that you believe in you can just go do it. No one's stopping you.

https://highscalability.com/the-architecture-twitter-uses-to...

The reality is people aren't gonna bother when Facebook/Instagram/Snapchat/Twitter/TikTok/Substack/etc is right there.