WhatsApp? Similar story, no AWS.

Well, WhatsApp was started by industry experts in scaling. If you're making a point about functional programming, I'd tend to agree but from a business perspective I'd look to why Netflix still uses AWS: https://www.quora.com/Why-does-Netflix-still-use-AWS

Still they place FreeBSD running on prem hardware in the ISP PoPs as a caching layer because it absolutely makes sense. https://papers.freebsd.org/2019/FOSDEM/looney-Netflix_and_Fr...

I think people tend to forget how often WhatsApp experienced outages in the early days.

This actually speaks volumes. Yes, people forget. Yes, it’s possible to suffer severe growing pains and still get acquired for $$$$. No, you don’t need to start with everything-AWS to ensure 99.9-whatever% uptime. People forget.

An $1B acquisition may have nothing to do with any colossal infrastructure. Was it indeed colossal?

I don't know what you'd consider colossal but the migration doesn't sound fun: https://www.wired.com/2014/06/facebook-instagram/

Why? Sharding and caching blob storage and activity feeds for Instagram type sites is among the easiest category of sites to scale.