Reddit shares jumped more than 14% on Friday after the S&P Dow Jones Indices said the social media platform would join the benchmark S&P 500 index, replacing AvalonBay Communities.Analysts at J.P. Morgan estimate index funds tracking the S&P 500 would need to ‌buy 16.7 ⁠million ⁠Reddit shares.That is nearly three times the stock's average daily trading volume of about ​5.98 million shares since the company went public in March 2024, according ​to LSEG data.Reddit was trading at $175.69, with shares worth more than $201 million changing hands by 09:32 a.m. ET.The stock has ​fallen more than 31% this year ⁠through Thursday's close ‌and is down more than 42% from ​its all-time high ​reached in September 2025.In its most recent ⁠earnings report, Reddit said choppy search-engine traffic hurt U.S. ​user growth in the second quarter, overshadowing an upbeat ​revenue forecast.The changes are effective before markets open on August 18.AvalonBay is being removed after agreeing in May to an all-stock merger with Equity Residential to form a rental housing company with an enterprise value of $69 billion. The companies have ‌said the deal is expected to close in the second half of 2026.The combined company, to be ​called Vivmark Residential, ​will remain in ⁠the S&P 500 after the merger is completed, S&P Dow Jones Indices said.Stephens analyst Melissa Roberts said new S&P 500 additions have historically ​outperformed the benchmark from announcement to inclusion, with the biggest gains coming the day after the announcement.After inclusion, however, they have typically given back some gains and lagged the benchmark over the next three months by about 2%.