It is staffed by curators who share uplifting content about social issues – "stuff that matters" – with a young online audience. And it notoriously uses the "curiosity gap" headline, a link that contains almost no information about the video or article you're about to click, but which pulls you in, regardless: "Watch The First 54 Seconds. That's All I Ask. You'll Be Hooked After That, I Swear" (a documentary about street kids making instruments out of garbage) or "His First 4 Sentences Are Interesting. The 5th Blew My Mind. And Made Me a Little Sick" (a polemical video about the US healthcare system).

The habit was enough to provoke one writer to dismissively conclude that "Upworthy is a site that posts viral videos with terrible headlines". Yet the site is doing something right. Upworthy's 100 most popular posts were viewed and shared more than 380m times during 2013, while the two-year-old site's record monthly visitors figure was in excess of 87m – around the same as the Guardian, which, as a newspaper, is approaching its 200th birthday.

Upworthy was launched in March 2012 by a group including Peter Koechley, a former managing editor of the satirical news site The Onion, and Eli Pariser, a co-founder of the social campaigning site Avaaz.

It had an explicit social purpose. At first it contained little other than a mission statement and links to articles and content relating to the Trayvon Martin case – the shooting of a Florida teenager that sparked protest across the US. Upworthy's curators are encouraged to share material with a message that will resonate with their target audience of progressive young readers, dubbed "millennials" or "Generation Y" by marketers and sociologists. The curators, many of them 20-somethings, believe social purpose is integral to the site's success in connecting with the demographic.

"A lot of general sites cover everything, and a lot of my friends – I have a lot of gay friends, minority friends – have a craving for that kind of content, and it's just an easier way to find it to go to Upworthy," says Joseph Lamour, who joined Upworthy from an entertainment news site last April.

Rossalyn Warren, 24, who joined the site the same month, says Upworthy supplements traditional media rather than replaces it. "I totally think that people of our generation, they definitely go to the Guardian, New York Times for hard-hitting journalism," she says, "but Upworthy [is] about visual, sharable content that appeals, because it's created to work on social platforms."

The site has recently started making a big deal out of its fact-checking controls. A blog from its copy chief stresses that "credibility is perhaps the most important trait of great curation".

This attitude is far from universal. Other viral content originators are known for spreading unverifiable information. Perhaps the most famous is FCKH8, a pro-gay rights online clothing vendor (with mixed reviews at best), which has a habit of finding heartwarming or shocking anonymous letters from parents to their closeted children, or from people to their bigoted family members. Efforts to find the writers of such letters generally fail. FCKH8's explanation – that they wish to stay anonymous – may be credible, but such content is, as a result, not always trusted.

There's set to be another uptick in Upworthy's quality, too. At the South by Southwest technology festival in Austin, Texas, Upworthy named a number of future journalistic collaborators, including the US-based investigative not-for-profit site ProPublica, which worked with the Guardian on some of the Edward Snowden NSA revelations.

ProPublica will collaborate with Upworthy on stories covering income inequality, they say. "We're big fans of what they've been doing. They've accomplished a lot in a short time," ProPublica's president, Richard Tofel, told Niemen Journalism Lab.

There's more science than you might suspect in creating Upworthy's much-maligned headlines. The trademark Upworthy headlines try to hook your interest without telling you much. For a time, they flew – but eventually disappointed users, who stopped going for them.

The headlines also backfired when other outlets tried them. CNN's Twitter account faced a wide backlash when it covered a stabbing with the headline: "14-year-old girl stabbed her little sister 40 times, police say. The reason why will shock you."

Upworthy noticed the curiosity-gap backlash before its critics, largely because of its relentless focus on testing its headlines to destruction. Each Upworthy article is given no fewer than 25 separate headlines, which are automatically tested to see which attracts the most hits.

So what would the headline on the Upworthy story so far be? The answer, Lamour suggests, isn't easy. "I would need time to write 25 headlines on that and then only pick one ... It would be a joke of some kind, I know that!"