Reddit shares rose 11% in extended trading on Thursday after S&P Dow Jones Indices said the social media company will join the S&P 500 on Aug. 18, replacing AvalonBay Communities Inc. Stocks added to major benchmarks often gain because index-tracking funds must buy them to reflect the new composition. The source also highlighted a related watch item: Reddit CEO Steve Huffman said after second-quarter results that investors are right to focus on U.S. user growth.