In his 2008 book, Click, online behaviour expert Bill Tancer declared that social media was overtaking pornography as the most popular destination on the internet. Those aged 18 to 24 in particular were replacing pornography use with more stimulating social networking pastimes. After the porn frenzy that was the first decade of the internet's life, users seemed to be finding more "sociable" ways to occupy their time.

Five years later and social media seems to be firmly ahead of pornography in the race for internet dominance – social networking sites make up four out of 10 of the world's most visited sites. Research from Pew's internet project suggests that 90% of 18- to 29-year-olds in the US use social networking and 71% of online adults are on Facebook.

But it's safe to say that pornography still remains popular. It's notoriously difficult to come by reliable statistics on porn use or the porn industry, but Pornhub – one of the biggest online providers – claims to have had more than 14.7bn visits in 2013, with more than 1.68m visits an hour.

However, the line between porn and social media is beginning to blur. From Fuckbook (a porn version of Facebook) to Pornostagram (a porn version of Instagram), to PornTube (a porn version of YouTube), online pornography websites are increasingly starting to behave like social networks – encouraging users to share, like, rate, comment, curate and even create content.

Traditional social media sites have always struggled with the "pornography problem" – the peculiar fact that whenever a means for people to share things online is created, people will start sharing explicit material. It only took four days after Twitter launched Vine for a pornographic video to creep to the top of its "Editor's Picks" list.

From his position in the digital startup industry in Barcelona, Christian Thorn noticed this tendency and spotted a business opportunity. "If people are putting that stuff up on social media, then they want a site that will allow them to do it," he says.

Thorn went on to found Pinsex – another addition to the social porn family – just over a year ago. It behaves like the photosharing website Pinterest – a virtual pinboard that allows users to collect images they like and follow others who have similar tastes. Pinsex does exactly the same, but with porn. In its first year 50,000 users have signed up and the site attracts 300,000 visits each day.

A tamer example of the material on the site – a photograph of a topless woman on a beach at dusk posted by a user called Nick – has had 124 repins and 382 likes. Many of the comments cannot be repeated for obvious reasons. "Pure perfection man," one user has commented. "Doesn't get much better than that," reads another. One user, who is apparently female, writes that she will follow Nick, if he follows her back.

"A few years ago nobody would have predicted that people would take pictures of their food and put them on Facebook," says Thorn. "People would have said: 'Who is interested in what I had for lunch today?'" Nowadays people are happier to share, he says, and that applies to porn too.

Pinsex users can be broadly split into two categories – those who just like to curate their own collections and enjoy other people's, and those who are creating their own images. "There are a lot of users posting amateur porn on the site and that porn might not be as beautiful and airbrushed – like you see in magazines or whatever – but it's very popular."

The socialisation of internet pornography has been noted with interest by academics. "Traditionally pornography was 'used', 'consumed' or whatever verb you wish to use, by people on their own," says Simon Lindgren, a professor of sociology and social media researcher at Umeå University in Sweden.

He is clear that today's online porn audience is no longer made up of "isolated masturbating loners", but of an interactive and creative group of critical audience members.

Sharif Mowlabocus, a senior lecturer in media studies at the University of Sussex, argues that the idea of pornography as an intensely antisocial activity is actually relatively new. "With the exception of certain times in history, pornography has actually always had a social dimension," he says. He points to the blue records of the 1920s and 30s – audio recordings of people having sex – which were often listened to in groups, and to the stag movies of the 1940s, which, again, were watched alongside other people in cinemas. "It was covert, but it was also social," he says.

The social side of pornography has perhaps been even more important in the history of oppressed sexualities: "In the 1980s there was widespread sharing of porn among particular groups, like the gay community or BDSM community. Those communities have a long history of developing social relationships around porn."

This all changed when the videotape came along. "The videotape took porn off the street, out of the movie theatres and into the intimate and domestic spaces of the home," says Mowlabocus.

'Whenever a means for people to share things online is created, people will start sharing explicit material' Photograph: Fabrice Lerouge/Getty Images/Onoky

If being sociable with your porn habits isn't new, then other things that come with the rise of social porn might be. Just as social media was credited as a catalyst for political revolution and a platform for hitherto unheard voices, some argue that the rise of social porn may democratise the world of pornography, providing space for alternative sexual desires.

Some argue that once the content is driven by the user, the porn landscape will look much more representative. "The rise of social media in a range of forms has had a crucial impact on the kinds of pornography that are available and how it is made available," says Susanna Paasonen, professor of media studies at University of Turku, Finland. "You get different production practices, different aesthetics and different politics."

"I started off studying mainstream online porn and now I don't think I know what that is any more – it's become so diverse," she says. "There are of course examples that are immediately recognisable as mainstream, but then on the same platforms you have all kinds of content that is a far cry from the mainstream."

Thorn says he wants to attract a diverse audience to Pinsex: eighty per cent of users are currently male and a tiny percentage are trans, says Thorn, but the site would like to attract a wider range of people – especially women. "We want to be a service for everybody because we're a service that's created by the users … In the old days people would say, 'but women don't like porn' and we can see now that that's not necessarily very true."

"I don't think that it's surprising that in some of these more social spaces – where porn is consumed, uploaded, distributed, commented on – we are beginning to see discussions about the alternative politics of pornography," says Mowlabocus.

But, although there is the opportunity and potential to democratise pornography, he says, it won't necessarily happen. "Those same types of sociality are still being used to uphold some very misogynist views," he says. "We need more than a technological platform to make those ideological shifts."

One thing seems clear – social porn isn't going anywhere. A Pew survey asked whether people thought the group referred to as Generation Y would continue to be "ambient broadcasters who disclose a great deal of personal information in order to stay connected" throughout their lives; 67% of those questioned thought so.

"It'll be interesting to see if those pornography networks seep out and connections start to be made with your personal email address," says Mowlabocus. But he suspects that social activity around online porn will always largely be done under the cloak of anonymity. "While porn has always had a social dimension, that social dimension has always been heavily policed."

It doesn't seem likely that your social porn activity will be recorded on your Facebook page, for example, as it is with other sites, including music-streaming site Spotify. "The people watching the stag films didn't tell their bosses and parents," says Mowlabocus.

"In the end, when you think about the people you're connected to on Facebook, there are a lot of people you wouldn't want to share your sexual desires with."