We write our backend mainly in Ruby and host on Heroku. We don’t use Rails but Sinatra, we use Postgres and Redis, and Sidekiq for background processes. We use a number of tools such as Librato, Skylight, Airbrake, and Logentries, and a host of gems, such as Twitter, Markio, and Pocket. Our crawlers are hosted on Joyent but we also use Mercury. For some high-performance tasks, we use Rust. We use CloudFront as our CDN and S3 for some parts.
Our mobile apps are hybrid at this point: we write the native parts (share extension, offline access) in Swift for iOS (using many Pods) and in Java for Android. We’ll go all native down the road.
We use git and store (most of) our repos on Bitbucket.