Still on firebase real time database, the code helps track when a user is online or not and also the last time a user is seen.

Presence.java

// since I can connect from multiple devices, we store each connection instance separately
// any time that connectionsRef's value is null (i.e. has no children) I am offline
final FirebaseDatabase database = FirebaseDatabase.getInstance();
final DatabaseReference myConnectionsRef = database.getReference("users/joe/connections");

// stores the timestamp of my last disconnect (the last time I was seen online)
final DatabaseReference lastOnlineRef = database.getReference("/users/joe/lastOnline");

final DatabaseReference connectedRef = database.getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    boolean connected = snapshot.getValue(Boolean.class);
    if (connected) {
      DatabaseReference con = myConnectionsRef.push();

      // when this device disconnects, remove it
      con.onDisconnect().removeValue();

      // when I disconnect, update the last time I was seen online
      lastOnlineRef.onDisconnect().setValue(ServerValue.TIMESTAMP);

      // add this device to my connections list
      // this value could contain info about the device or a timestamp too
      con.setValue(Boolean.TRUE);
    }
  }

  @Override
  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled at .info/connected");
  }
});