A breakdown of how the "For You" feed decides who sees your post. Based on X's open code. If you are just getting started, this is your entry point. No jargon, but nothing important is skipped.
When you publish a post, X does not show it to everyone immediately. First, the post goes to a small group: your followers and people whose behavior looks similar to your audience. Then the system watches what those people do: whether they pause, reply, repost, or scroll past. If the reactions are good, the post is distributed further to people who do not follow you. If the reactions are bad, the post dies.
The first 30-60 minutes decide everything.
When a person opens X, the feed is assembled from two sources:
Posts from people they follow. These are all fresh posts from their subscriptions. They are stored in memory and served instantly.
Posts from unfamiliar authors. These are posts from people they do not follow, but whose behavior is similar to people they already interact with.
Unfamiliar posts receive a score penalty because the system prefers showing "their own" authors. The exception is new accounts with little history: they are shown more unfamiliar posts so they can find their interests faster.
Inside X there is one large AI model: a transformer based on Grok. For each viewer, it predicts: "If I show this post to this person, what is the probability that they will like it, reply, repost, dwell on it, close it, or report it?"
For each action, the model outputs a number from 0 to 1. Then those numbers are multiplied by the weights of the corresponding actions and added together. This creates the final score of the post for that specific viewer.
The higher the score, the higher the post appears in the feed. Then the system takes the top-N posts with the highest scores and builds the final feed.
Viewer actions that help you, from the most valuable to the least valuable.
Very valuable signals: