We also took an estimated 30bn selfies last year and we load 300 hours of video on to YouTube every minute. How is it affecting the way we see the world?

Who are the Snapchat generation? They are the new global majority. They are young – under 30 and under 21 in the developing world. More people on Earth now live in cities than rural areas, and they are networked. Roughly half the world has access to the internet today. By 2020, companies such as Google predict at least 5 billion people will be online.

The new majority are trying to understand the world they live in by taking and sharing visual images in extraordinary numbers. Americans take more photographs every two minutes than were taken worldwide in the entire 19th century. Three hundred hours of YouTube video are uploaded every minute. On Snapchat, alone – an app founded in 2011 and banned in China – 700m photos are posted every single day. In 2014, 1tn photographs were taken, more than a quarter of all previously existing photos.

There is a new “us” using the internet to share all these images that is different from any “us” that print or media culture has seen before. Anthropologist Benedict Anderson has described how print culture created “imagined communities”: readers of, say, a specific newspaper felt they had something in common with other readers they had never met. Above all, modern nations were shaped as imagined communities. From Scotland to Catalonia to Quebec, that mindset is less powerful now. From the new feminisms to the idea of the Occupy movement’s “99%”, the Snapchat generation are reimagining how they belong and what that looks like. They are using new media forms to begin to change the world in unanticipated ways.

There have been previous “frenzies of the visible”, to quote film historian Jean-Louis Comolli. In 1895, the first moving images were shown in a Paris cafe by the Lumière brothers. Their one-minute dramas showed staged scenes such as workers leaving their factory in Lyons, carefully avoiding looking at the camera. In the same year Wilhelm Roentgen discovered the x-ray and used it to take a picture of his wife Anna’s hand. Suddenly, humans could experience moving images and see the hidden interior of their own bodies for the first time. A year later, the Lumières made a film of a train arriving at a station. Legend has it that the audience ran from the pictures of the train in terror. It didn’t happen. Changes in the visible happen because people try to make them and anticipate them.

Today’s expanded visual media are producing a generation with better peripheral and central vision, learned from video games. Scientists call this “probabilistic inference”, meaning decisions we make based on incomplete information, such as when we drive a car. In the 1990s, psychologist Daniel Simons and his student Christopher Chabris created a video test for attention. Subjects were asked to count how often one of the two basketball teams being shown passed the ball. As this simple action unfolded, a person in a gorilla suit walked across the screen. Half the people watching did not notice, due to what Simons called “inattentional blindness”. Young people today mostly tend to see the gorilla. Formerly, we were trained to concentrate on one task, like a factory worker. Mostly, if not exclusively, we did. Now we are supposed to pay attention to distractions such as email notifications and, mostly, if not exclusively, we do.

The first signature of this new visual culture is the now infamous selfie. The selfie is the culmination of a long democratisation of the self-portrait. Once, portraits and self-portraits were the preserve of artists and their wealthy patrons. Photography expanded that field almost as soon as it was invented: French photographer Hippolyte Bayard made Self-Portrait as a Drowned Man in 1840, the first true selfie because it could be copied. The combination of smartphones with front-facing cameras and social media has meant that selfies have been ubiquitous since 2010. “Selfie” was the Oxford Dictionaries’ word of the year in 2013, used 17,000% more often than the year before. Google estimated that 30bn selfies were taken in 2014.

Media commentary has mostly castigated the selfie as narcissistic. But Narcissus, whatever you think of him, spent all his time looking at himself; he did not make copies of his image to share with all his friends. And the gender is wrong. The website SelfieCity has shown that most selfies are taken by women – as many as 82% in Moscow. Already a cliche, the selfie is not important in itself, but it shows us that the new majority is inventing ways to see itself. These work best when there is a formal limit. Like the 140-character limit on Twitter, Vines have exploded because the six-second limit of the video posts tests the imagination. First used for silly stunts, Vines have become a form of film-making and are now used to document protests and as a form of micro-reporting.

All these new visual media are helping to create social change. Beginning with the Arab spring, social media have changed the shape of politics. According to the Pew Research Center, 85% of African Americans aged 18-29 own a smartphone, several points higher than their white counterparts. In Ferguson, Baltimore and Texas, phones have been used to document police violence. These assaults, which have long persisted out of sight of the mainstream media, are now widely discussed and, finally, officers are being indicted. In the Irish referendum campaign on same-sex marriage, viral online videos combined with old-fashioned canvassing to create a surprising cross-generational majority.

To borrow the term coined by South African photographer Zanele Muholi, we are seeing the rise of a new “visual activism”. Muholi uses her work to claim the right to call herself a “black lesbian”. She is caught between the constitution of her country that formally guarantees rights to LGBT people and the daily reality of violence and sexual assault against them in the townships. Her work has come to global prominence, with a current exhibition entitled Isibonelo/Evidence at the Brooklyn Museum in New York. Using such evidence, visual activism is set to shape how the new majority wish to be seen by themselves and by others, when the state will not, or cannot, represent them.

• Nicholas Mirzoeff’s How to See the World is published by Pelican.