It has also been remarkably successful in using Facebook’s algorithm to get topics that matter onto people’s feeds, in a time when Facebook is becoming increasingly crucial to publishers. The dominant social network in America, Facebook claims 57 percent of American adults as members and 73 percent of teens between 12 and 17. More importantly, 64 percent of users visit the site daily. Facebook’s power over publishers surfaced this fall, when the company announced changes to its News Feed that would place “quality” content, like journalism, in more prominent places in people’s feeds. Traffic to publishers from Facebook referrals shot up. Sites in the BuzzFeed network released a report showing that Facebook was now sending them 3.5 times more traffic than Google.

Upworthy has been able to vault content past the wall of Facebook’s algorithm with just a few tweaks. Before Pariser and Koechley founded Upworthy, reworked the headline of a video about a young man testifying on behalf of gay marriage. The original post, “Zach Wahls speaks about family,” earned one million hits. But when retitled it “Two Lesbians Raised A Baby And This Is What They Got,” it received 17 million. In the world of Facebook, 11 words, posted at the top of the same video, could mean an additional 16 million readers for a video, not about cats, but about marriage equality.

“The big overarching story of Upworthy, and the world of journalism should be very heartened by this, is that there are tens of hundreds of millions of people that care intensely about the most important issues of society,” Koechley told me when I asked him how reporters should respond to the site.

But those people only care about important topics within a specific context. Upworthy’s most popular posts of 2013 include heavy subjects—gender, body image, cancer—but the videos themselves are, as the site’s cheery name suggests, often oriented around uplifting messages. The cancer story isn’t about research funding; it’s about a 17-year-old cancer patient recording an album. Their top “racial profiling” story is an alarming, advertisement-like stunt showing that an African-American kid trying to steal a bike gets dealt more skepticism than a white kid. At South By Southwest, Carr summarized criticisms of the site as, “you’re not making the news more interesting, you’re making the news sound more interesting.” Which begs the question—is Upworthy really changing the world for better by altering what it sees?

Upworthy’s seemingly effortless ability to transform ordinary stories and videos into hits actually hinges on a whole lot of research about what drives us to click and to share. It also hinges on a time-honored workforce: twentysomethings. Titled “curators,” this remote crew trolls the Web for items they find personally arresting. They come from a variety of backgrounds, usually not journalism. “Lots of journalists have been so ingrained to write in the opposite way and come in so allergic to analytics,” Sara Critchfield, Upworthy’s editorial director, told me via email. “Our voice and methods can be hard for them to pick up.”

But as Upworthy’s staff knows, it takes more to get people to share than it does to click. The selection process is about exclusion—a curator may post only five or six items a week. In December, for instance, Yahoo’s news network published 130,000 articles; The New York Times produced 7,000. Upworthy posted 246. Items are “packaged” with a Facebook-ready image and a headline that’s been rigorously A/B tested in a focus group. (Curators, infamously, must produce 25 headline options for each post.) The most promising videos are posted to sites in Upworthy’s “partner network,” whose thousands of fans act as lighter fluid to a gently stoked flame. In February Upworthy announced a shift in the way it measured its audience from “page views” to something called “attention minutes”—counting not just readers but how much time they spend on each piece of content. It’s a collective measurement of not just who is viewing a piece of content, but how deeply it’s sinking in. A recent video by Human Rights Watch, consisting of interviews with people released from North Korean prisons, has garnered over four minutes of attention per user. “Which means that for each person that is spending 10 seconds on the video, there is someone sticking around for eight minutes,” says Koechley.

Alexis Sobel Fitts is a senior writer at CJR. Follow her on Twitter at @fittsofalexis.