Reddit's index seat forces about $2.9bn of passive buying in a stock down 31% this year

Reddit joins the S&P 500 before trading opens on August 18, taking the seat vacated by AvalonBay Communities, and J.P. Morgan estimates the switch obliges index funds to buy 16.7 million shares ^[3][1]. The stock is down more than 31% this year as of Thursday's close, which is the whole point: index demand and business performance are decided by different processes ^[4].

The mechanical part is easy to size. Since its March 2024 debut, Reddit has traded an average of 5.98 million shares a day ^[2], so the forced purchase is roughly 2.8 normal sessions of volume compressed into a rebalance ^[6]. At Friday's level near $175.69 ^[5], that is about $2.9 billion of price-insensitive buying ^[18]. The market did what it usually does with that information in advance: shares rose more than 11% on Friday after S&P Dow Jones Indices confirmed the move ^[7], with one intraday quote showing a 14.55% gain at $181.12 ^[8], and more than $201 million had changed hands by 9:32 a.m. ET ^[9] - roughly 19% of a full day's average volume in the first two minutes ^[19].

Melissa Roberts at Stephens says additions have historically outperformed the index between announcement and inclusion, with the largest move the day after the news ^[10]. Her second point is the one that matters for anyone buying now: those gains tend to dissipate once the stock is in, and additions have trailed the benchmark by about 2% over the following three months ^[11].

The underlying business is not the problem. Reddit reported second-quarter net income of $253 million on revenue of $805 million, up 61% year over year and its eighth consecutive quarter of growth above 60% ^[12], a net margin of about 31% ^[20]. It raised its revenue outlook, but choppy search-engine referrals hit US user growth late in the period ^[13]. Chief executive Steven Huffman told analysts that "visibility into referral traffic remains low," adding that Google's AI overviews have not yet replaced the traditional search links Reddit depends on ^[14]. That dependency, rather than the ad line, explains why the stock sits more than 42% below the high it set in September 2025 ^[15].

AvalonBay's exit is unrelated housekeeping. The apartment landlord agreed in May to an all-stock merger with Equity Residential carrying an enterprise value of roughly $69 billion and expected to close in the second half of 2026 ^[16]. S&P Dow Jones Indices said the merged company, to be called Vivmark Residential, will retain AvalonBay's spot in the index once the deal closes ^[17].

Two things to watch. First, whether the historical post-inclusion drag shows up here, given how much of the buying was front-run before the rebalance ^[11][7]. Second, referral traffic: Huffman's own framing is that Reddit cannot yet see it clearly ^[14], and that is the variable that decides US user growth. Worth noting the room Reddit is entering - stocks linked to AI now account for about 45% of the S&P 500's market value, according to Cryptopolitan, which makes the benchmark more sensitive to a handful of firms ^[21].