code: public void loginWithFacebook(final String appId, final String accessToken, final SimpleLoginAuthenticatedHandler completionHandler) {
      if(appId == null || accessToken == null) {
        handleInvalidInvalidToken(completionHandler);
      }
      else {
        HashMap<String, String> data = new HashMap<String, String>();
        data.put("access_token", accessToken);
  
              loginWithToken(Constants.FIREBASE_AUTH_FACEBOOK_PATH, Provider.FACEBOOK, data, completionHandler);
          }
    }
docs: Login to Firebase using a Facebook token. The returned FirebaseSimpleLoginUser object will contain pertinent
  Facebook data accessible with getThirdPartyUserData().
  
  @param appId Facebook app id.
  @param accessToken Access token returned by Facebook SDK.
  @param completionHandler Handler for asynchronous events.