X just gave every user 75 custom timelines powered by Grok. The announced feature is personalization. The actual mechanism is training data acquisition at a scale Meta paid $72 billion for last year. Here's what changed at the feed level. The For You algorithm ranked every post against one global engagement signal. Creators competed in the same homogenized feed. Now each user pins up to 75 topics, and every pinned timeline runs its own Grok-sorted ranking tied to what you engage with inside…