Public filings and earnings materials for Klaviyo’s 2024 results show that by the end of calendar year 2024 the company crossed a $1 billion annualized revenue run rate, which is effectively how public SaaS companies proxy for ARR when they don’t disclose ARR explicitly.

Key points:

• In Klaviyo’s official FY 2024 earnings release (filed with the SEC and distributed via BusinessWire), CEO Andrew Bialecki states that they “had a very strong finish to the year, crossing a $1 billion revenue run rate” as they closed 2024, with Q4 2024 revenue of $270.2M and full‑year 2024 revenue of $937.5M. (sec.gov) Annualizing the Q4 revenue ($270.2M × 4 ≈ $1.08B) is consistent with that claim.
• The Q4 2024 earnings call summary likewise notes that Q4 revenue of ~$270M put Klaviyo at a ~$1B revenue run rate, with full‑year revenue of ~$937M, again indicating that by late 2024 they were operating at roughly a $1B annualized subscription revenue level. (news.futunn.com)
• Independent write‑ups summarizing the same results repeat that Klaviyo “achieved a $1 billion revenue run rate” for FY 2024. (tipranks.com)
• A third‑party data site estimates Klaviyo’s full‑year 2024 revenue at ~$937M and trailing‑twelve‑month revenue at about $1.01B by March 31, 2025, which is consistent with the company having exited 2024 near or above a $1B recurring run rate. (canvasbusinessmodel.com)

While some external estimate models pegged end‑2024 ARR around ~$903M, those are not based on Klaviyo’s own disclosed run‑rate commentary and were issued before final numbers. (sacra.com) The official language from the company is that they crossed a $1B revenue run rate as they finished 2024, which is exactly the kind of milestone Sacks was pointing to with “probably going to be at a billion in [recurring] revenue next year.”

Given:

• the prediction said “roughly” $1B,
• Klaviyo’s own 2024 disclosures say they crossed a $1B annualized run rate during that year, and
• there is no contrary official data showing they were far short of that threshold,

the most reasonable judgment is that Sacks’ prediction came true in substance.