In just a few years, it has gone from an upstart student news website at Cambridge University to a venture capital-backed international media organisation making major strides into campuses across the UK and the US.

And, with the launch of a new national site this month, the Tab is looking beyond the graduation ceremony and trying to keep hold of its readers well into their advertiser-beloved 20s.

Its success, its young bosses say, is built on its two-pronged approach: a team of 30 editorial staff building on the work of an army of mainly student contributors posting stories on its 100-or-so campus-based and national websites.

The Tab is one of very few for-profit university media organisations. Its revenue comes from advertisers seeking access to the readership those writers and editors can reach. And they can reach an increasingly large number.

According to editor-in-chief Jack Rivlin, its various sites had 3.2 million unique visitors in November 2015 and are on course to increase that figure by more than 50% this month.

It is a business model that convinced the US firm Balderton Capital to invest $3m (£2m) in the Tab in December to help it expand across the Atlantic. It also relies heavily on the unpaid labour of thousands of people.

While the Tab is not alone in that respect and is trialling performance-related pay for its best writers this month, some – including former Tab contributors and the National Union of Journalists (NUJ) – accuse it of exploitation.

Not so, says its executive editor Joshi Herrmann, who argues that very few of the student contributors ever ask to be paid. He says they are remunerated in other ways: the Tab helps them with editorial and legal guidance, as well as helping to prepare them for a future career in journalism.

“We are between a platform and a publisher. Our most dedicated people are probably treating us more like a publisher and getting involved in editorial decisions and legal calls. But the rest are really just posting in the way that they do on 10 different networking sites.

“You wouldn’t ask Twitter for money, you wouldn’t ask Instagram, you wouldn’t ask Facebook. Those companies provide amazing platforms that people really like that help them get their voices out there.”

Herrmann, a former Evening Standard journalist, says he has asked his staff how often students mention money in their dealings with the Tab. “It’s incredibly rare,” he says. “I think we’re one of the few media groups at the moment that’s employing people, not laying people off. Hopefully, our model will keep on giving jobs to journalists and editors.”

Under the trial, contributors whose work is shared on Facebook 1,000 times will be given £35. That figure rises in increments up to £700 for 50,000 shares, although Herrmann admits it is unlikely anyone will reach that. The editors who run each of the the Tab’s university based sites only need to get half those shares to be paid.

In the first eight days of the trial, seven of the Tab’s 3,000 contributors crossed the lower threshold, Rivlin says, adding: “The aim is to provide a bonus to people who drive big audience, but it’s not a key motivator or core reason to get involved.”

The Tab defines the payments as prize money and, consequently, as exempt from minimum wage law. It’s a not uncontroversial attitude.“If they can afford to pay any contributors, then they should be paying all their contributors,” says contributor Rinna Keefe, who objects to a sliding scale that leaves some without any pay.

“[They should introduce] either a basic rate for everybody – or if you wanted to incentivise, maybe a sliding scale of bonuses above a basic rate,” says Keefe, whose unpaid Tab writing credits include an article about the evils of unpaid internships. Yes, she says, she gets the irony.

Eva Nelson, another unpaid Tab contributor says: “I do think they are riding on the exploitation because, despite writing for them, I do believe that any work has to be remunerated if it generates profit.”

Both, however, say they recognise that few, if any, students sign up with the expectation of being paid.

The NUJ’s freelance organiser John Toner has a more concise view. “If work is worth publishing then it is worth paying for.” He adds: “There is never any excuse for asking anyone to work for free. It is exploitation, pure and simple. The question I would put to students is this: ‘Why would you value a publisher who does not value you?’”

And yet, the Tab presents itself as being on the side of the oppressed student. Shortly before speaking to the Guardian, Herrmann says, he was dealing with a “big, prestigious US university” that was threatening to “kick one of our writers off an elite athletics team because she wrote something that they didn’t like”. He will name neither the school, nor the student in order to protect his writer.

But he does stress the chilling effect the approach of that institution – and others – is having on writers. “I’m having to deal with an administrator at an elite university who won’t let this girl write what she wants. It’s not defamatory, it’s not illegal, it’s just a piece about doing her sport.”

The Tab’s ability to protect its writers comes from its maligned business model, say Rivlin and Herrmann. Most student papers are dependant on either their student union branch or the university for funding.

Because the Tab is financially independent of both, it can hold them to account more effectively, as well as protecting its writers, Rivlin and Herrmann argue.

“Journalists and institutions should have adversarial relationships but watching university administrators threaten people’s places at universities … makes me sick,” says Rivlin.

Its proximity to students has helped the organisation break exclusives, albeit as well some dodgy stories they later had to apologise for. And what of its bigger potential competitors, such as Buzzfeed, the Guardian and others?

“We don’t really worry about big media companies, they don’t understand young people, and young people don’t trust them,” Rivlin says.

He also says he is proud of the Tab’s ability to help students be fearless in the faces of “bullshit companies who are trying to fuck them over”. He doesn’t agree with those who suggest the Tab in some ways is starting to look like one of them.