Hi there!
Just as @oallouch, we’re using Parse in a large french App (social network for places sharing) with more than a million users.
We use the JS SDK, the iOS SDK and the android SDK.
We use Parse-Server on nodes with MongoDB.
And we are hosted on AWS ElasticBeanstalk, with MongoDB Atlas and a connector to ElasticSearch
@oallouch we should talk 
I would love to know more about your Pub/Sub usage & conf, and the database options you use