Twitter Inc. shares rallied in the extended sesion Monday after S&P Dow Jones Indices announced the microblogging platform would join the S&P 500 index Twitter shares rose 3.8% after hours, following a 3.4% rise to close at $37.88 in the regular session. Twitter shares are up 58% for the year, compared with a 2.7% rise in the S&P 500. Twitter will replace Monsanto Co. which is being acquired by Bayer AG on Thursday. On the S&P 100 index Netflix Inc. will replace Monsanto. Netflix shares were up 0.2% after hours.