There was a period around 2010 where developers all declared sql dead, and started playing with nosql solutions. My personal experience at the time was, most people didn't have Facebook's scaling problems, and they created terrible tech debts as a result.
Eventually most people remembered why SQL was so great, and they added Redis if they needed a cache. The Ruby people though still liked those NoSQL stuff, and I'll never understand them.
I’ve been working in Ruby that whole time, and Postgres is like the right arm of the prototypical Ruby app. We did have a fad where Mongo was used at my company in the early ‘10s, and indeed we had little choice but to do rewrite that part. We actually had an amazing intern do that one year.
I started in PHP land 20+ years ago, moved into Ruby for the next 10, and now am firmly in JS/Node land. I have used Postgres nearly the entire time. It's my experience that you will never go wrong picking Postgres as your primary data store. Love that DB.
MySQL backed, in turn, by a log structured storage engine (RocksDB). But the they reason they do that is storage space efficiency rather than availability or performance.
Given how many users they have I suppose they know a thing or two about scale.