The Facebook News Feed has long been a mystery (and sometimes annoyance) for the social network's 1 billion users. Why do pictures of puppies and babies always seem to pop up, and posts from that person you met five years ago on a trip to New York?

Well, this week Facebook lifted the curtain, ever so slightly, on how it populates the News Feed in a special event at its headquarters in Menlo Park, California.

1500 stories

Lars Backstrom, engineering manager for Facebook's News Feed, explained how his team determines which posts are more pertinent.

On average, each user has 1500 possible stories that can appear in their News Feed, though only 20 per cent make the grade.

Initially, Facebook would manually tweak the News Feed based on feedback from users, adding a few more photos here and there. These days a complex algorithm, which Backstrom says is one of the most complex moving parts of Facebook, determines which stories are shown.

“News feed is one of only places where Facebook is doing things on the scale of complexity of what Google is doing or Bing is doing in search,” he explained.

Relationships

In short, Facebook populates the News Feed based on relationships.

For example how many times you've commented on a particular user's posts, how many friends you have in common, location, the type of posts that are typically Liked and shared and so on.

Each post is assigned a score around these parameters and the higher the score, the better the chance of appearing in the News Feed.

Relevance

Until now the most current posts were pushed to the top of the News Feed, even if they had a lower score than older, more relevant posts.

Now Facebook has tweaked their algorithm so more relevant stories, even if posted earlier in the day, will still appear with the “new” posts at the top of the feed.

Backstrom says the change, which has been rolled out on Facebook.com and is soon coming to mobile devices, has resulted in more engagement. On average there's been a 13 per cent increase in stories read and a 5 per cent increase in Likes, shares and comments. 

While this news does not affect how paid advertisements appear in the News Feed, it may help brands further tweak their Facebook content strategy and foster more engagement.

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