code: public String auth_getSession(String authToken)
      throws FacebookException, IOException {
      if (null != this._sessionKey) {
        return this._sessionKey;
      }
      JSONObject d = (JSONObject) this.callMethod(FacebookMethod.AUTH_GET_SESSION, 
                                                  new Pair<String, CharSequence>("auth_token", authToken.toString()));
      this._sessionKey = (String) d.get("session_key");
      Object uid = d.get("uid");
      try {
        this._userId = ((Long) uid).intValue();
      } catch (ClassCastException cce) {
        this._userId = Integer.parseInt((String) uid);
      } 
      if (this.isDesktop()) {
        this._sessionSecret = (String) d.get("secret");
      }
      return this._sessionKey;
    }
docs: Sets the session information (sessionKey) using the token from auth_createToken.
  
  @param authToken the token returned by auth_createToken or passed back to your callback_url.
  @return the session key
  @throws FacebookException
  @throws IOException