It is actually using the steem json rpc protocol. So if steemit would be down this would still work.  Currently I use  `wss://this.piston.rocks ` as backend. It is planned that you can change the url to your liking enabling anybody to use their own or other servers. You can read more about the actually implementation here: https://steemit.com/android/@razem-io/building-an-android-app-for-steemit