title: Firebase Auth: Get users by email or phone number
body: <p>I am building an <strong>Android</strong> App where I am using different ways for the user to register himself, like Email/Password, Phone, Google, Facebook, Twitter. I also want the users to be able to add one another as their contacts.  </p>
  
  <p>If I was using Email and Google only, then it would've been easy to implement this as I could store the User's Email in the DB and use that to identify different users and create Relations between them.  </p>
  
  <p>But now, the problem is that a User could register using their Phone Number as well and Facebook accounts could also be registered using Phone Number only. So, I can't just use the Email only as User's unique ID.  </p>
  
  <p>What are some of the ways I could achieve the desired result i-e create unique user IDs in the DB so that Users could add one another as Contacts?</p>