Jay Yarow explains:

Facebook's News Feed has always been something of a mystery. 

Some posts show up in the feed, some don't. The feed has become overrun with all sorts of content and Facebook uses an algorithm to manage it. 

But no algorithm is perfect. 

CEO Mark Zuckerberg found this out when the top story in his feed was a co-worker's birthday. At the same time, his niece had been born.

According to a new Steven Levy story in Wired, when Zuckerberg saw his co-worker's birthday ranking ahead of his niece's birth, he asked for a tweak to Facebook's News Feed algorithm. 

The tweak was that if the word "congratulations" is in the comments, a post gets a boost.  This makes sense because congratulations is the No. 1 thing people say in response to huge events, like an engagement, a graduation, a new baby, or a new job — exactly the kind of stuff that Facebook wants to surface.

"We’re testing a lot of things every day to figure out what’s going to make the most engaging feed," Zuckerberg tells Levy. "We use quantitative metrics that measure likes and comments and clicks and shares and other activities to see if a story is good, but we also have qualitative systems so that people can reorder a feed to tell us what they thought were the most important things."

For more on how it works, go to Wired »