https://www.theverge.com/2018/1/26/16937088/facebook-news-feed-explainability-ai

One reason Facebook struggles to earn our trust is because at the individual level, no one at the company can tell us why we’re seeing what we’re seeing in the News Feed. The company can talk about the content of the feed in general terms — mostly posts from friends and family, ranked by how close Facebook believes you to be with them — but were an engineer to browse your feed alongside you, they couldn’t explain why the posts appeared in the exact order they did.

A few years ago I was interviewing Chris Cox, who leads product across the company, and asked something about my feed I had always wanted to know. Sometimes I would open Facebook after being away for an hour or so and the News Feed would show me one or two posts I had already seen. Was that an effort to get me to add a comment? Did Facebook think I’d be more likely to share something after I saw it a second time? No, Cox said. That was just a bug.