> And, if you remember in the early days, Twitter's scalability was absolutely atrocious - the "Fail Whale" was an emblem of Twitter's engineering headaches.

That's because Twitter was:

1) A monolith

2) Written in Ruby

They started splitting components up in to specialised made-for-purpose components using Scala atop the JVM, and scaling ceased being a big issue. The problems they ran into couldn't be solved by horizontal scaling. There wasn't any service that AWS offers even today that would have helped with those engineering challenges.