public Facebook login(Callback callback, Class<? extends Activity> clazz) {
    if(!(context instanceof clazz)) {
      System.out.println("context is not a instance of activity or fragment");
      return null;
    }
    this.callback = callback;
    request = new Session.OpenRequest((clazz)context);
    request.setPermissions(permissions);
    request.setCallback(callbackFacebook);
    session = new Session(context);
    Session.setActiveSession(session);
    session.openForRead(request);
    return this;
  }