Services that require a short iteration cycle, or any proof-of-concept work.
Services that require high concurrency and all the speed.
Backing data store.
Testing of Ruby APIs, Chef cookbooks, and builds of iOS apps.
The iOS version of our native apps.
The Android version of our native apps.
Configuration management for any services not provided by AWS.
Testing of Java APIs.
Redis & memcached for our APIs.
Pretty much all the things.
Framework for our front-end web apps.
Our de facto language, when all other considerations are equal.