Donna Tam/CNET

MENLO PARK, Calif. -- 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 will 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 a new blog dedicated to explaining News Feed changes, in hopes of quelling the concerns of businesses.

The engineering team meets each week to discuss how they can improve News Feed, Backstrom said. With the bumping stories method, it took engineers three months to build the infrastructure to support it. The team then rolled it out to 80 percent of Facebook employees to test the new ranking. Among the test group, commenting, liking, and sharing on friends' posts went up 5 percent. For business pages and users with followers, those engagement marks went up 8 percent. The test group was also reading 70 percent of stories, up from 57 percent, according to Backstrom.

Facebook

Another change, called "last actor," to the algorithm factors in your last 50 interactions with each of your friends. Both these features have already launched. Facebook is actively bumping stories on the Web, but is still working on making it live on mobile. The company is using the last actor method on both Web and mobile.

Backstrom also explained another method, that didn't pan out. He said the company tested posting stories from one user chronologically despite the score of each post. Engineering has plans to go back to the drawing board on the feature because it resulted in less engagement among users, according to Backstrom.

The company held a similar press event in November after businesses started accusing Facebook of changing its algorithm to make fewer users see their pages. At the time, Facebook said it did do a major tweak in September, but the new algorithm is simply more accurate and factors in how likely a user will not like a post. Cathcart said then that some brands saw even more engagement with users.

Facebook is constantly tweaking its algorithm because the News Feed is the main feature of its network, not to mention the way it makes advertising dollars. The company announced a major redesign of the product in March, promising a bigger, bolder News Feed.

News Feed ads did well in Facebook's last quarter, with particular flourish for mobile. To ensure readers don't see irrelevant ads or content -- which makes for unhappy customers and no ad clicks -- the company lets users hide posts and ads. Facebook recently started polling some users about why they hide certain News Feed items.

Updated, 11:15 a.m. PT : Added more information.