We write our backend mainly in Ruby, using a number of gems. We don’t use Rails but Sinatra. Data is stored in a Postgres database, and we use Redis and Sidekiq for background processes. Our app is hosted on Heroku, images on Cloudinary, assets etc. are delivered via CloudFront, and we use Amazon S3 for some parts. Some high-performance tasks are written in Rust. Also, we use a number of tools such as Librato, Skylight, Airbrake, and Logentries.
Our mobile apps are hybrid at this point: a web app with some native parts written in Swift (iOS, using many Pods) and Java (Android). We plan to go all native down the road.
We use git and store (most of) our repos on Bitbucket.