More than 4.5 million researchers have signed up for ResearchGate, and another 10,000 arrive every day, says Madisch. That is a pittance compared with Facebook’s 1.3 billion active users, but astonishing for a network that only researchers can join. And Madisch has grand goals for the site: he hopes that it will become a key venue for scientists wanting to engage in collaborative discussion, peer review papers, share negative results that might never otherwise be published, and even upload raw data sets. “With ResearchGate we’re changing science in a way that’s not entirely foreseeable,” he says, telling investors and the media that his aim for the site is to win a Nobel prize.

The company now employs 120 people, and last June it announced that it had secured US$35 million from investors including the world’s richest individual, Bill Gates — cash that came on top of two earlier rounds of undisclosed investment. “It was really a head-scratcher when we saw that,” says Leslie Yuan, who heads a team working on networking and innovation software for scientists at the University of California, San Francisco. “We thought — who are these guys? How are they getting so much money?”

Publishers are worried that the sites could become public troves of illegally uploaded content. In late 2013, Elsevier sent 3,000 notices to Academia.edu and other sites under the US Digital Millennium Copyright Act (DMCA), demanding that they take down papers for which the publisher owned copyright. Academia.edu passed each notice on to its users — a decision that triggered a public outcry. One researcher who received a take-down request did not want to be named, but told Nature: “I hardly know any scientists who don’t violate copyright laws. We just fly below the radar and hope that the publishers don’t notice.”

These concerns are not unique to large social networks, says Price; the same issue surrounds content posted in universities’ online repositories (to which Elsevier also sent some DMCA notices last year). “This is really part of the wider battle where academics want to share their papers freely online, whereas publishers want to keep content behind a paywall to monetize it,” he says, noting the nuance that many publishers allow researchers to upload the final accepted version of a manuscript, but not the final PDF. He has seen fewer take-down notices this year.