Flask drives our APIs, both the Website APIs and the majority of the REST Messaging APIs
All incoming requests pass through our high redundancy load balanced nginx instances, no matter if it's static resources for the websites, API calls via uwsgi or TCP stream-based services.
Python is (together with C++) the main language of GatewayAPI.com, we use it for the bulk of our APIs.
All data not yet commited to PostgreSQL, lives in multiple redis instances. We are heavy users of lua in redis to enable custom queue behavior.
The most latency sensitive parts are written in C++. Due to our interconnected services architecture, we use either Python or C++ for each service, with the performance critical parts being C++14.
Almost all company communication happens in slack, it's the beating heart of OnlineCity.
Our platform is based on interconnected services with a custom RPC protocol based on ZeroMQ and inspired by ZeroMQs LPP/MDP protocols.
PostgreSQL is our datastore, used for long term storage and analytics. But not active data, such as messages enroute.
We use CF for DNS hosting, since their AnyCast DNS provides the best latency in the business, and they support DNSSEC + IPv6. We don't use the CDN or website optimizations.