In April, Shi Tao, a journalist working for a Chinese newspaper, was sentenced to 10 years in prison for "providing state secrets to foreign entities". He had passed details of a censorship order to the Asia Democracy Forum and the website Democracy News.

The pressure group Reporters Without Borders (RSF) was mystified by the ease with which Mr Tao had been caught. He had sent the message through an anonymous Yahoo! account. But the police had gone straight to his offices and picked him up. How did they know who he was?

Last week RSF obtained a translation of the verdict, and there they found the answer. Mr Tao's account information was "furnished by Yahoo Holdings". Yahoo!, the document says, gave the government his telephone number and the address of his office.

So much for the promise that the internet would liberate the oppressed. This theory was most clearly formulated in 1999 by the New York Times columnist Thomas Friedman. In his book The Lexus and the Olive Tree, Friedman argues that two great democratising forces - global communications and global finance - will sweep away any regime which is not open, transparent and democratic.

"Thanks to satellite dishes, the internet and television," he asserts, "we can now see through, hear through and look through almost every conceivable wall. ... no one owns the internet, it is totally decentralised, no one can turn it off ... China's going to have a free press ... Oh, China's leaders don't know it yet, but they are being pushed straight in that direction." The same thing, he claims, is happening all over the world. In Iran he saw people ogling Baywatch on illegal satellite dishes. As a result, he claims, "within a few years, every citizen of the world will be able to comparison shop between his own ... government and the one next door".

He is partly right. The internet at least has helped to promote revolutions of varying degrees of authenticity in Serbia, Ukraine, Georgia, Kyrgyzstan, Lebanon, Argentina and Bolivia. But the flaw in Friedman's theory is that he forgets the intermediaries. The technology which runs the internet did not sprout from the ground. It is provided by people with a commercial interest in its development. Their interest will favour freedom in some places and control in others. And they can and do turn it off.

In 2002 Yahoo! signed the Chinese government's pledge of "self-regulation": it promised not to allow "pernicious information that may jeopardise state security" to be posted. Last year Google published a statement admitting that it would not be showing links to material banned by the authorities on computers stationed in China. If Chinese users of Microsoft's internet service MSN try to send a message containing the words "democracy", "liberty" or "human rights", they are warned that "This message includes forbidden language. Please delete the prohibited expression."

A study earlier this year by a group of scholars called the OpenNet Initiative revealed what no one had thought possible: that the Chinese government is succeeding in censoring the net. Its most powerful tool is its control of the routers - the devices through which data is moved from one place to another. With the right filtering systems, these routers can block messages containing forbidden words. Human-rights groups allege that western corporations - in particular Cisco Systems - have provided the technology and the expertise. Cisco is repeatedly cited by Thomas Friedman as one of the facilitators of his global revolution.

"We had the dream that the internet would free the world, that all the dictatorships would collapse," says Julien Pain of Reporters Without Borders. "We see it was just a dream."

Friedman was not the first person to promote these dreams. In 1993 Rupert Murdoch boasted that satellite television was "an unambiguous threat to totalitarian regimes everywhere". The Economist had already made the same claim on its cover: "Dictators beware!" The Chinese went berserk, and Murdoch, in response, ensured that the threat did not materialise.

In 1994 he dropped BBC world news from his Star satellite feeds after it broadcast an unflattering portrait of Mao Zedong. In 1997 he ordered his publishing house HarperCollins to drop a book by Chris Patten, the former governor of Hong Kong. He slagged off the Dalai Lama and his son James attacked the dissident cult Falun Gong. His grovelling paid off, and in 2002 he was able to start broadcasting into Guangdong. "We won't do programmes that are offensive in China," Murdoch's spokesman Wang Yukui admitted. "If you call this self-censorship then of course we're doing a kind of self-censorship."

I think, if they were as honest as Mr Wang, everyone who works for Rupert Murdoch, or for the corporate media anywhere in the world, would recognise these restraints. To own a national newspaper or a television or radio station you need to be a multimillionaire. What multimillionaires want is what everybody wants: a better world for people like themselves. The job of their journalists is to make it happen. As Piers Morgan, the former editor of the Mirror, confessed, "I've made it a strict rule in life to ingratiate myself with billionaires." They will stay in their jobs for as long as they continue to interpret the interests of the proprietorial class correctly.

What the owners don't enforce, the advertisers do. Over the past few months, AdAge.com reveals, both Morgan Stanley and BP have instructed newspapers and magazines that they must remove their adverts from any edition containing "objectionable editorial coverage". Car, airline and tobacco companies have been doing the same thing. Most publications can't afford to lose these accounts; they lose the offending articles instead. Why are the papers full of glowing profiles of the advertising boss Martin Sorrell? Because they're terrified of him.

So instead of democracy we get Baywatch. They are not the same thing. Aspirational TV might stimulate an appetite for more money or more plastic surgery, and this in turn might encourage people to look, for better or worse, to the political systems that deliver them, but it is just as likely to be counter-democratic. As a result of pressure from both ratings and advertisers, for example, between 1993 and 2003 environmental programmes were cleared from the schedules on BBC TV, ITV and Channel 4. Though three or four documentaries have slipped out since then, the ban has not yet been wholly lifted. To those of us who have been banging our heads against this wall, it feels like censorship.

Indispensable as the internet has become, political debate is still dominated by the mainstream media: a story on the net changes nothing until it finds its way into the newspapers or on to TV. What this means is that while the better networking Friedman celebrates can assist a democratic transition, the democracy it leaves us with is filtered and controlled. Someone else owns the routers.