Your point 2 sounds like there were additional factors that could have influence the reliability besides the load (they didn't simply migrate to FB infra but also switched to Signal).
> The broadcasting is important, yes, but it is very heavily biased towards reads over writes, so something like Cloudflare would solve 99% of the load.
There's push-notifying millions of devices within seconds after a celeb or a major news source tweets. There's tracking view and engagement stats on that in realtime. There's making sure a tweet is not available to any of those within seconds after it's been deleted or moderator. There're separate back-office apps for moderating that firehose of content. And that's just what I can see from the outside. An e2ee instant messenger with size-limited chat groups doesn't even come close.
Please don't say "just stick a CDN on top of it and you are 99% there", it's embarrassing (and not to twitter). This will maybe get you 80% there if your goal is "a microblogging platform" but not even 20% if your goal is being both the go-to news source and shitpost forum for people worldwide reliably working even in sensitive times and emergencies. Twitter used to be a microblogging platform back when it had much fewer employees and you'd see a fail whale regularly even as it had much fewer active users, in recent yeas it's a completely different beast and saying increased headcount is unrelated is amusing.
