[From www.News.CNet.com]

Notice older stories popping up on your News Feed? Or, seeing more posts from one friend rather than another? It’s all part of Facebook’s top-secret ranking system.

Facebook offered a deep dive into its its News Feed ranking algorithm on Tuesday, expounding on why it moves up old stories and how it picks which stories it thinks you want to see.

Lars Backstrom, the engineering manager in charge of News Feed ranking, explained how Facebook sorts through the “tens of thousands” of potential posts users put on Facebook each day. While there is a median of 1,500 potential stories that a user can see daily, Facebook inserts about 300 based on an algorithm that guesses how interested you w

ill be in a post by factoring users’ reactions to previous posts and the users. Each post is given a score and placed depending on that score. The more likes and comments people make, the more data Facebook has to work with.

“It’s a very personalized thing, it depends on your relationship to the person, what things you’ve liked in the past,” Backstrom said, echoing CEO Mark Zuckerberg’s often mentioned “personalized newspaper” comparison.

One way Facebook has done this is a method called “story bumping,” which moves up older stories that you might not have seen because it was “below the fold” of your News Feed.

“It’s really hard for users to get back to old things, you have to scroll back to things you’ve already seen,” Backstrom said.

In explaining these changes, Facebook hopes to shed some light on why it makes the changes it does. Past changes have sparked controversy. Product Manager Will Cathcart said Tuesday’s event was timed with the rollout of the bumping stories method, but the company plans to proactively talk about changes to the news feed in the future. Facebook has even started