title: Android connection with other phone
body: <p>I want to create a simple social app. On this app, users can communicate via the internet. It will be a simple connection - notification sends from one to other users. (ex. user A sends a notification to user B. User B see a notification from user B).</p>
  
  <p>How the best achieve this communication? Is Google Cloud Messaging is the best solution? How can I calculate costs?</p>