In addition to what @newjersey said, you also need to handle push notifications, which in the context of a mobile phone aren't cheap at all. This is because you need infrastructure and because keeping connections open drains your battery and making it work well is wasted developer effort. This is why it is better to use whatever is available, like GCM on Android, APNS on iOS, or maybe Pushy (see pushy.me) or whatever Amazon does if you want distribution on alternative stores.