Another mentions-based trends site https://db-engines.com/en/ranking_trend that shows Postgres uptrend. Most surprising thing to me there is that MongoDB is still going up.

Is anyone using Mongo in a large volume production system?

Lots of major players are using it: https://www.mongodb.com/who-uses-mongodb

We use it for large volumes of machine learning features and it's been amazing.

Major players using it doesn't mean it's used for large-scale projects. I've seen several companies testing it but none of them uses it for any of their core production systems.

MongoDB has improved a lot in the last few years.