title: Firebase Facebook login check if user exist
body: <p>I have an facebook login system which works with firebase but I want to check if user exist on my firebase (i don't want to add it, just want to make sure if he exist because I want to redirect user to another page to complete its profile, once its done I'll want to send it to firebase).</p>
  
  <p>I just need to check if user exist on my db. Here is the code that I try but it returns nil error and it automatically add user to firebase.</p>
  
  <pre><code>let credential = FIRFacebookAuthProvider.credentialWithAccessToken(FBSDKAccessToken.currentAccessToken().tokenString)
  
  
  FIRAuth.auth()?.signInWithCredential(credential) { (user, error) in
          // ... 
  
  }
  </code></pre>