An Apartment REIT Merger Just Put Reddit in the S&P 500

AvalonBay's Takeout Made the Seat, Not Reddit's Quarter

The catalyst is mechanical, and it is worth being precise about why. S&P Dow Jones Indices announced after Thursday's close that Reddit replaces AvalonBay Communities in the S&P 500 before the opening bell on Tuesday, August 18. The vacancy exists because Equity Residential is acquiring AvalonBay; the combined company gets renamed Vivmark Residential and keeps its own seat. Reddit was not promoted for a great quarter. A REIT merger opened a slot and Reddit was the name on deck.

What that buys Reddit is a forced, price-insensitive bid. Roughly $13 trillion tracks the index, and at a float-adjusted weight of about 0.05%, one estimate puts the required passive buying on the order of $6–7 billion of RDDT against a ~$30 billion market cap. It also makes Reddit only the second pureplay social platform in the index after Meta, which matters for the mandates that screen on membership rather than on fundamentals.

The tape responded the way that setup implies. RDDT closed the regular session at $158.12 and printed up to $177.08 in extended trading, about 12% higher. The HIP-3 perp marked $174.10 — a live, continuous price for an event that landed when the cash market was already shut.

The Index Is Buying a Stock Down 30% on the Year

This is not a momentum name catching a tailwind. RDDT went into Thursday down roughly 31% year to date, and the damage came in two identifiable chunks.

On July 22, shares fell 9% on a report that Reddit was weighing whether to renew Google's access to its content for AI training — the $60 million-a-year arrangement signed in 2024. Then on July 30, Reddit posted Q2 revenue of $805 million, up 61% year over year, with advertising at $762 million and daily active uniques of 130.3 million — an eighth consecutive quarter of 60%-plus growth — and the stock dropped about 10% anyway. The reason was one line from the call: CEO Steve Huffman said search referrals were choppy in the quarter and traffic got more volatile late, pointing at Google's AI Overviews answering queries in place rather than sending clicks. US daily active uniques grew 6%. That is the number the market is trading, not the 61%.

One detail sharpens the valuation question. Reddit repurchased 1.5 million shares in Q2 at an average price of $157.57 — within a rounding error of Thursday's $158.12 close. Management's own cost basis is right where the stock sat before the announcement. Index funds now have to source stock roughly 10% above it.

The Bid Has an Expiry Date

The classic index-inclusion trade is not what it was. Greenwood and Sammon's "The Disappearing Index Effect" documents that abnormal announcement returns for S&P 500 additions have decayed toward zero as liquidity improved and the flow became easy to anticipate. An 11–12% pop is still a real repricing, but the mechanism behind it is finite and scheduled: the bulk of index execution lands at the close on Monday, August 17, the last session before the effective open. After that print, the buyer with no price sensitivity is gone.

What is left is the fundamental argument nobody has resolved. Whether Reddit renews with Google, what AI Overviews does to the referral funnel, and whether US user growth reaccelerates off 6% — none of that changed on Thursday. The index add is a flow event stapled onto an unchanged story.

For perp traders the structural detail is the timing. The announcement hit after the equity close, so the RDDT perp was the only continuous venue pricing it — and it repriced into a thin book, with about $1.25 million of 24-hour volume on this specific market. At 10x, the gap risk around Monday's rebalance runs in both directions, and the basis between the perp mark and the cash print is where the mismatch shows up first.