Facebook/Gmail/Twitter Connection with Unity3D

I have a Unity3D application who needs registration.

The user needs to register in the application. He can simply enter an email adress and a password. Those are used to access the application.

But i want to offer the possibility to connect with other existing account like facebook, gmail, twitter…

Is it possible ? Everything must be in the Unity App but can use http request.

Concerning Unity/Facebook, I used a very good tutorial:

It uses the JavaScript Facebook Api and the usefull functions :

<UNITY> Application.ExternalCall("YourJavaFunction");
<JS>    getUnity().SendMessage("YourGameObject", "YourUnityFunction", yourdata);