code: public String auth_getSession(String authToken) throws FacebookException,
                                                           IOException {
      if (null != this._sessionKey) {
        return this._sessionKey;
      }
      Document d =
        this.callMethod(FacebookMethod.AUTH_GET_SESSION, 
                        new Pair<String, CharSequence>("auth_token", authToken.toString()));
      this._sessionKey =
          d.getElementsByTagName("session_key").item(0).getFirstChild().getTextContent();
      this._userId = Integer.parseInt(d.getElementsByTagName("uid").item(0).getFirstChild().getTextContent());
      if (this._isDesktop) {
        this._sessionSecret =
            d.getElementsByTagName("secret").item(0).getFirstChild().getTextContent();
      }
      return this._sessionKey;
    }
docs: Call this function to retrieve the session information after your user has
  logged in.
  @param authToken the token returned by auth_createToken or passed back to your callback_url.