Nicolas Carlson explains:

In November, viral-content-for-a-cause site Upworthy posted insane traffic numbers, reaching almost 90 million people around the world, according to Quantcast.

Then, in December, Facebook announced a change to the algorithm it uses to determine what kinds of updates ("stories") users see in the News Feeds. In a blog post, Facebook said it wanted to feature more "high quality" content and fewer "meme photos."

That same month, Upworthy's traffic dropped 25% – reaching 67 million people around the world between December 1 and December 31.

Then, in January, Upworthy traffic dipped even lower – to 48 million people.

That's a 46% traffic decline in two months. 

Carlson speculates that it's because Facebook hates clickbait headlines like this one:

Upworthy cofounder Peter Koechley doesn't think Facebook is out to nuke Upworthy:

He thinks Upworthy's December and January traffic is only suffering in comparison to a November because "November was a huge crazy spike of a month, driven by some off the charts posts."

"We expected the next couple months traffic would be lower."

Koechley also says that when Upworthy's traffic spiked so much in November, he and his cofounder Eli Pariser used the headroom to "raise the bar" on the kinds of stories the site's editors produce.

Specifically, Upworthy decided to publish fewer stories that are "just personally uplifting" and don't deal with "income inequality, gender discrimination, or racial injustice."

Koechley points out that more people visited Upworthy in December 2013 and January 2014 than they did in October 2013 — so the overall trend is still positive. He's right. Upworthy had 5 million readers a year ago and has 50 million today.

But he may be wrong about Facebook.

Note that the same decline did not happen to BuzzFeed:

Buzzfeed, unlike all those other sites, buys traffic from Facebook. Buzzfeed's business model is to create advertorials on Buzzfeed.com and then get traffic to these advertorials by buying Facebook ads. 

If that's the reason, then the message Facebook is sending isn't so much that it wants "high quality" content for its News Feed. It's that if you are a media company, and you depend on Facebook for your traffic, you better make sure Facebook is benefiting from your existence.

(To this, a Facebook spokesperson says: "Organic News Feed ranking is not impacted at all by ads. We try to show people the things they will find the most interesting based on what and who they interact with, not who spends money on Facebook.")

More likely, it's that, unlike all those other sites, Buzzfeed employs several respected journalists who publish hard news and smart analysis. Maybe, in the eyes of Facebook executives, this makes all Buzzfeed content "high quality" and therefore News Feed-worthy. Other sites, including Slate, The Atlantic, and Business Insider, have also invested in journalism and continue to benefit from Facebook traffic.

Here is the BuzzFeed traffic chart: