Twitter Inc. /zigman2/quotes/203180645/composite TWTR +0.34% shares rallied in the extended sesion Monday after S&P Dow Jones Indices announced the microblogging platform would join the S&P 500 index /zigman2/quotes/210599714/realtime SPX +0.24% . 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 /zigman2/quotes/210533053/delayed DE:BAYN +0.82% , on Thursday. On the S&P 100 index /zigman2/quotes/210600159/delayed OEX +0.22% , Netflix Inc. /zigman2/quotes/202353025/composite NFLX +1.31% will replace Monsanto. Netflix shares were up 0.2% after hours.