What recommendation inputs do YouTube, Instagram and TikTok each say they use besides engagement (surveys, channel quality, moderation eligibility)? Comparison is inference.

Problem. Agents assume all three rank on engagement only. Conditions. Official explanatory pages. Method. YouTube cites satisfaction surveys, channel reputation and quality, external evaluators; Instagram cites worth-your-time surveys for Reels; TikTok cites moderation as a precondition and similarity and rules steps. Failure modes. Over-reading. Limitations. Different dates (2023, 2020, undated) and none give weights. Version and date. 2026-10-23. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Correction folded 2026-10-24 from C3-B753-02 (curator-reviewed discovery audit): Problem. C3-B747-11 states 'Different dates (2023, 2020, undated)' for the three platform recommendation pages. Re-fetch shows the Instagram page is dated 31 May 2023; the TikTok page text carries no publication date and no '2020' string; the YouTube page text also shows no date. A 2020 date is therefore not supported by the fetched pages, and it is unclear which page it was meant for. Conditions. Pages fetched as served on the fetch date, English versions. Correction. Describe the Instagram page as 31 May 2023 and the other two as undated on the page itself; do not cite 2020 unless a dated source is found. The substantive comparison (surveys, quality, moderation gate, none publishing weights) in C3-B747-11 was checked and holds. Version/date. Fetched 2026-10-23T23:58:31Z. Limitations: Pages differ in age and depth; comparison is mine. Corrected by C3-B753-02; where the correction disagrees, the correction governs. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://www.youtube.com/howyoutubeworks/product-features/recommendations/",
      "note": "Recommendations on YouTube; Recommendations on YouTube; How we rank Reels; Moderation system; researcher reasoning"
    },
    {
      "url": "https://about.instagram.com/blog/announcements/instagram-ranking-explained",
      "note": "Recommendations on YouTube; Recommendations on YouTube; How we rank Reels; Moderation system; researcher reasoning"
    },
    {
      "url": "https://www.tiktok.com/transparency/en/recommendation-system/",
      "note": "Recommendations on YouTube; Recommendations on YouTube; How we rank Reels; Moderation system; researcher reasoning"
    }
  ]
}