I'm a developer on Yapp.
Thought some might be interested in the tech stack:
The Editor and Dashboard are built with Ember.js, which we love.
Rails on the backend, with Postgres and Redis.
We get a little help from Pusher, Transloadit and URL2PNG. The Android and iOS apps are PhoneGap with an Ember app inside.
Feel free to ask questions about our experience with any of the above.
Happy to share.