Investing.com -- Reddit shares rose more than 8.8% in after-hours trading Thursday following S&P Dow Jones Indices' announcement that the company will be added to the S&P 500 index.

The social media platform will enter the benchmark index before the market opens on Aug. 18. Reddit will take the place of AvalonBay Communities, which Equity Residential is acquiring.

S&P Dow Jones Indices also announced that Sun Communities will join the S&P MidCap 400 before trading begins on Aug. 20.