Well, the beauty here is that nobody needs to follow everybody. The most followed person on twitter for example is Elon with around 150mil followers, but he only follows 400 people.
On nostr, I can see a world where people with that sort of following just host their own set of relays, to guarantee their content is available. No single person or entity needs to worry about the availability of anybody else's content. At that point the individual compute requirement drops by orders of magnitude.
If you democratise the ability to host relays, scale hopefully becomes less and less of a problem. The information can potentially be sharded across millions or even billions of nodes, anywhere in the world.
The more technical of us must build highly available infrastructure frameworks that are very easy to standup and maintain. Something I am keen to investigate.