Twitters architecture was a bad joke. Many to many communication in a scalable manner has been a solved problem from decades: you federate. You assign users to buckets, you assign buckets to servers. You route messages like you'd route e-mail. Been there, done that. The federation does not need to be outwardly visible.

Twitters problem was a too centralized architecture not Rails.

And I say that as someone who at the time hated Rails and who still hates Rails. I find it bloated and over-complicated. It may even have led them to make bad architectural choices because of how it was structured.

But they still did make bad architectural choices, and they fixed those choices at the same time they moved off Rails.