Store Facebook user in Firebase

Hello I'm new to Android development. I am currently trying to build a basic android application with Google's Firebase as its backend system.

I have a "Login with Facebook" button, whenever this button is clicked it opens up a dialog where the user enters his facebook credentials in order to get access to the application. When that's done, I want to store his email, name and id in Firebase if it's his first time logging in, if it's not his first time logging in I want to update his information.

I made a class called User (POJO) that I will use to represent this data. My problem is that I don't seem to know where to put the code that stores this information. Here's my MainActivity class:

public class MainActivity extends AppCompatActivity {

    //Declare our view variables
    private LoginButton mLoginButton;
    private CallbackManager mCallbackManager;
    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;
    private FirebaseDatabase mDatabase;
    private DatabaseReference mDatabaseReference;

    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FacebookSdk.sdkInitialize(getApplicationContext());
        setContentView(R.layout.activity_main);

        //Initialize callback manager
        mCallbackManager = CallbackManager.Factory.create();

        //Assign the views to the corresponding variables
        mLoginButton = (LoginButton) findViewById(R.id.login_button);

        //Assign the button permissions
        mLoginButton.setReadPermissions("email", "public_profile");

        //Create instance of database
        mDatabase = FirebaseDatabase.getInstance();
        mDatabaseReference = mDatabase.getReference();

        //Assign the button a task
        mLoginButton.registerCallback(mCallbackManager, new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                Log.d(TAG, "facebook:onSuccess:" + loginResult);
                handleFacebookAccessToken(loginResult.getAccessToken());
            }

            @Override
            public void onCancel() {
                Log.d(TAG, "facebook:onCancel");
            }

            @Override
            public void onError(FacebookException error) {
                Log.d(TAG, "facebook:onError", error);
            }
        });

        // Initialize Firebase Auth
        mAuth = FirebaseAuth.getInstance();

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                //Get currently logged in user
                FirebaseUser user = firebaseAuth.getCurrentUser();

                if (user != null) {
                    // User is signed in
                    Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());

                    // Name, email address
                    String name = user.getDisplayName();
                    String email = user.getEmail();

                    // The user's ID, unique to the Firebase project. Do NOT use this value to
                    // authenticate with your backend server, if you have one. Use
                    // FirebaseUser.getToken() instead.
                    String uid = user.getUid();

                    //Create user
                    final User loggedIn = new User(uid, name, email);

                    mDatabaseReference.child("users").child(loggedIn.getId()).setValue(loggedIn);

                    mDatabaseReference.child("users").addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            // This method is called once with the initial value and again
                            // whenever data at this location is updated.

                        }

                        @Override
                        public void onCancelled(DatabaseError error) {
                            // Failed to read value
                            Log.w(TAG, "Failed to read value.", error.toException());
                        }
                    });

                } else {
                    // User is signed out
                    Log.d(TAG, "onAuthStateChanged:signed_out");
                }
            }
        };
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        // Pass the activity result back to the Facebook SDK
        mCallbackManager.onActivityResult(requestCode, resultCode, data);
    }

    @Override
    public void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthListener != null) {
            mAuth.removeAuthStateListener(mAuthListener);
        }
    }

    //If user successfully signs in the LoginButton's onSuccess callback method
    // get an access token for the signed-in user, exchange it for a Firebase credential
    // and authenticate with Firebase using the Firebase credential

    private void handleFacebookAccessToken(AccessToken token) {
        Log.d(TAG, "handleFacebookAccessToken:" + token);

        AuthCredential credential = FacebookAuthProvider.getCredential(token.getToken());
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        Log.d(TAG, "signInWithCredential:onComplete:" + task.isSuccessful());

                        // If sign in fails, display a message to the user. If sign in succeeds
                        // the auth state listener will be notified and logic to handle the
                        // signed in user can be handled in the listener.

                        if (!task.isSuccessful()) {
                            Log.w(TAG, "signInWithCredential", task.getException());
                            Toast.makeText(MainActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
    }
}

Here's my User class:

public class User {
    private String id;
    private String name;
    private String email;

    public User(){

    }

    public User(String id, String name, String email) {
        this.id = id;
        this.name = name;
        this.email = email;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

}

Thank you for your time