Last October the number of photographs hosted on Flickr.com passed the four billion mark. By the time you've finished reading this column, several thousand more will have been uploaded to the site, each automatically resized and allocated its own unique URL for copying and pasting into emails, text messages and blog posts. Flickr has become the world's shoebox, a place where anyone can store their pictures, or put on an exhibition. This is good news for most of us, but it is driving professional photographers to share the slough of despond with the increasing number of print journalists who likewise feel that their occupational world is being torn apart by technology.

Last week, the New York Times carried an elegiac piece about the plight of professional snappers under the headline "For Photographers, the Image of a Shrinking Path". Human interest was provided by the contrast between a young graduate of a photojournalism course, who sees his career options vaporising, and a 40-year-old mother of six who uploads her digital pictures to Flickr from which some of them are licensed by the stock-photography agency Getty Images for a few bucks a time.

Because Flickr is so prominent, it'll get most of the blame for the destruction of yet another venerable profession. But in fact the rot had set in long before the site launched in February 2004. The main culprit was the idiot-proof digital camera, which enabled almost anyone to take a decent photograph, or at any rate one that was accurately exposed, in focus and sharp — and to delete it and try again if it hadn't turned out right.

Digital cameras had a powerful "levelling-up" impact on amateur photography. Once upon a time, only professionals could consistently deliver images that were technically excellent. And even then, analogue technology often let them down. I've just been looking through a book of Henri Cartier-Bresson's collected portraits, for example. Out of 94 images, only 66 approach contemporary standards of sharpness and focus. That doesn't mean that most of them aren't memorable pictures; but it does illustrate how digital technology has levelled the playing field.

Professional photography comes in various genres, each of which is being affected in different ways by changing technology. At the top end lies photojournalism and fashion photography. These are being undermined by the decline in print media, the rise of video and the advertising recession, and may or may not recover depending on what happens to the print publishing business.

Lower down the artistic scale we find the bread-and-butter of professional photography: routine capture of low-profile news events, product photography, pre-shot stock photography and other kinds of humdrum stuff. All of these businesses are being undermined by Flickr and its ilk. "Let's say that 20 years ago," writes one industry blogger, "a newspaper in New York was running a story about Hollywood and the editor wanted a picture of the Hollywood sign. Rather than fly one of their photographers out there, put him up in a hotel, and pay for his meals they would go to a company that specialised in stock photography. They would pay the company a fee for the licence to use their photo in the story and everyone is happy".

And now? "The newspaper can log on to a site like Flickr, find an image they like, and run it in their paper for free with just giving the owner credit (unless the licence is non-commercial). So now you have a happy editor who saved more money and a happy photographer getting exposure for his work. But you also have an unhappy stock photography business owner who has just been cut out of the deal."

What's happening to professional photography is just one instance of "the mass amateurisation of publishing", to use a phrase coined by the cultural critic Clay Shirky, who has no time for elegies for vanishing worlds. He sees our current angst as just the latest instalment of a reaction that's been going on since Gutenberg upended the apple-cart. "Most of the arguments made against the printing press were correct, even prescient," he says. "Readily available translations of scripture did destroy the Church as a pan-European institution. Most of the material produced by the new class of publishers was flyweight. Scribes did lose their social function. And so on."

Something similar is happening now, Shirky claims. The internet, like printing before it, is producing "a staggering volume of new material, some good but most flyweight. It too is upending the role of traditional gatekeepers and destroying the older economics of scarcity. And it too is leading to a cottage industry of hand-wringing: 'Why can't we just get a little bit of internet, but keep most things the way they were?'"

Why indeed? Trouble is: nostalgia, like photography, ain't what it used to be.