Social feed ranking

Design the ranking service for a social feed with 400M daily active users where each session must rank ~500 candidate posts down to a scrolling feed in under 200ms p95. Ranking must balance multiple objectives (predicted like, comment, share, watch-time, and a 'don't show me this' negative signal) into a single ordering, respect integrity demotions, and adapt within a session as the user scrolls. Cover the model architecture, how you combine objectives, the serving path, and how you measure whether a ranking change is actually good.