Reddit Inc. will join the S&P 500 index effective prior to the start of trading on August 18, replacing AvalonBay Communities Inc. S&P Dow Jones Indices made the announcement on August 13 as part of an off-cycle change prompted by the acquisition of AvalonBay by Equity Residential, another S&P 500 member.

Reddit shares rose more than 11 percent in after-hours trading following the news. The stock continued to climb on Friday as investors anticipated increased buying from index-tracking funds.

The addition marks Reddit as only the second pure-play social media company in the S&P 500, joining Meta Platforms. Other social media firms such as Pinterest and Snap remain outside the index due to smaller market capitalizations. Twitter, now known as X, was previously included before its acquisition in 2022.

Reddit went public in March 2024 under the ticker RDDT. Its inclusion reflects the company's growth in user engagement and advertising revenue since its debut on Wall Street. Index additions often trigger automatic purchases by passive investment vehicles that mirror the benchmark.

Sun Communities Inc. will separately join the S&P MidCap 400 on August 20, replacing Webster Financial Corp. These changes highlight ongoing shifts in major equity indexes driven by mergers and corporate performance.