code: public static final void saveSession(Session session, Bundle bundle) {
          if (bundle != null && session != null && !bundle.containsKey(SESSION_BUNDLE_SAVE_KEY)) {
              ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
              try {
                  new ObjectOutputStream(outputStream).writeObject(session);
              } catch (IOException e) {
                  throw new FacebookException("Unable to save session.", e);
              }
              bundle.putByteArray(SESSION_BUNDLE_SAVE_KEY, outputStream.toByteArray());
              bundle.putBundle(AUTH_BUNDLE_SAVE_KEY, session.authorizationBundle);
          }
      }
docs: Save the Session object into the supplied Bundle. This method is intended to be called from an
  Activity or Fragment's onSaveInstanceState method in order to preserve Sessions across Activity lifecycle events.
  
  @param session the Session to save
  @param bundle  the Bundle to save the Session to