At Parse, we decided to build our own push notification service so we could support older clients, people who preferred not to use C2DM (GCM was not yet available), and devices like the Kindle Fire which do not have access to C2DM/GCM. The client-side library is a long-polling TCP socket, which Java supports reasonably well out of the box. It's implemented as a true kernel-blocked long-poll, rather than a polling loop, to save battery life. This is the best practice whether written by Parse, Google, Apple, or Amazon.

The backend does the heavy lifting. We keep costs down by building as much on top of open source platforms as possible. We use a variety of open source technologies at Parse, including Rails, Resque, EventMachine, MongoDB, Redis, etc.