Business Context

Meta's Reels team has seen a sudden decline in sharing on Facebook Reels over the last 3 weeks after a ranking and composer update. Leadership wants a structured root-cause investigation because sharing is a key engagement signal tied to distribution, creator growth, and downstream session depth.

Metric Scenario

Weekly Reels share rate fell from 8.4% to 6.9% (-1.5 pp, -18%) while Reels impressions stayed flat at ~1.2B/week. Reels opens are down only 2%, but the share button tap-through rate dropped from 11.0% to 8.7%. The decline is concentrated on Android (-24%) versus iOS (-9%). New users (<30 days since signup) are flat, while existing users declined 21%. Average watch time per Reel is down from 14.2s to 13.6s, and 7-day Reels retention moved from 41% to 39%. A PM asks whether this is a content-quality issue, a UI regression, a ranking change, or a measurement bug.

Requirements

1. Define the primary metric precisely, including numerator, denominator, and edge cases.
2. Break the metric into a diagnostic funnel to isolate where the drop is occurring.
3. Identify the highest-priority cuts of data you would review first and explain why.
4. Propose 4 plausible root-cause hypotheses and how you would validate each.
5. Recommend immediate actions if the issue is caused by product behavior versus instrumentation.
6. Name guardrail metrics you would monitor while fixing the issue.

Data Available

• reels_impression_log
• reels_watch_events
• reels_share_events
• reels_composer_events
• ranking_experiment_exposure
• app_version_release_log
• user_profile_dim
• creator_content_dim