Another day, another story of the forces of order hounding an innocent citizen for making innocuous remarks on Twitter. This week’s target was Rob Marchant, a centrist Labour supporter, who was chatting online with a few comrades. They all opposed Lutfur Rahman, the sly and to my mind thoroughly unappetising mayor of Tower Hamlets.

Labour had expelled Rahman, a frontman for Islamic Forum Europe, after he ran against the official Labour candidate to become mayor. Unlike many of the conformists and appeasers on the London left, Marchant and his friends believed that it is the job of leftists to oppose the religious right. Not everyone agrees with that admirable sentiment. The supporters of Ken Livingstone are constantly agitating for Labour to readmit Rahman: in part because they like anyone, even religious reactionaries, who are against “the West”; in part because they have the Tammany Hall politician’s respect for the ethnic bloc vote Islamic Foreign Europe can mobilise.

Musing on this theme, Marchant joked to his friends that if Labour were to readmit Rahman they would just have to kill themselves. ‘I will load the revolver and we can all take turns,’ were his precise words.

You can probably guess what happened next. Hugh Muir the Guardian’s diarist, called Marchant. Rahman’s people were accusing him of threatening to kill him, and the Met were investigating the ‘death threat’. At least Muir had the decency to mention that Marchant was joking about suicide rather than assassination. No such delicacy restrained the London Evening Standard and the Left Futures blog, both of which left their readers with the distinct impression that Labour members were threatening to gun down Rahman.

An understandably dazed Marchant wrote

Let’s follow the logic here. Were I a would-be assassin, it seems firstly particularly odd in that I would choose to make such a threat in a publicly accessible way as Twitter. Secondly, the fact that it was only addressed to my own Twitter followers, meant that anyone wanting to make something of this would have had to actively seek out this tweet. In order to be an even vaguely credible threat, it would, surely, need to have been addressed to Rahman himself. It was not.

It looks like the fuss has blown over, and that London’s ever watchful police service has found more pressing matters to investigate.

The wider point remains that the Web in general and Twitter in particular provides indelible evidence that the malicious, the prissy and the vengeful can use against you, on a scale we have not seen before.

Older people lament the decline of reticence. They condemn the young for making public exhibitions of themselves on the Web, and add the popularity of reality TV and confessional memoirs to their lamentations on the decline of modern manners.

But it is too simplistic to think new technologies get their users into trouble by making the private public. Rather, they make what was public but virtually unknowable available to everyone willing to search for incriminating evidence. Say 20 years ago, you had got blind drunk. Your friends take pictures of you in a deplorable state. The police arrest and prosecute you. The magistrate fines you for being drunk and disorderly, and a reporter records the verdict with a paragraph or two in the local paper. Society would have punished you in a public hearing. In theory, you have a stain that might stay with you for the rest of your life. In practice, strangers need never know. The friends’ pictures would go in an attic rather than on the Web; the newspaper cutting and police record would be buried in filing cabinets.

Before the Net, the courts convicted a friend of mine for possessing marijuana. Because he was the child of famous parents, the story made the national press. He grew up to become a jobbing news reporter moving from office to office. In every one, he sneaked into the library and ripped up the reports of his conviction. By the time he had finished, it might as well never have happened. The public event was effectively private.

A few years later, a friend on the Independent was also convicted of possessing drugs. The most malicious man on the paper was, as so often, the religious affairs editor. (Holiness corrupts, in my experience, and absolute holiness corrupts absolutely.) He wrote a column in the Church Times berating the sinfulness of Independent journalists – we drank too much, slept with people we should not have done and so on – and put it online. He did not name anyone apart from the woman with the drugs’ conviction, whom he seemed to hate with a passion. She has an unusual name. And for the next 15 years, every time she applied for a job, prospective employers have googled it and found out about her record. She’s lost several promotions as a result.

What was once an obscure misdemeanor her enemies could never find was traceable via a search engine in seconds. Twitter is particularly dangerous because of the power of the written word. Newspaper and book publishers have always known that they are more likely to be sued for libel than broadcasters because print is a permanent record. It feels more solid and damning than broadcasts which disappear into the ether. The trouble is that people write on Twitter and Facebook as if they are talking to friends. They behave as if they do not have to mind their manners and bite their tongues. I like that. I don’t want the freedom to communicate to be curtailed. But there is no doubt that the illusion that no one can listen in, leaves incautious users wide open to attack.

The Marchant case makes my point. If he and his friends had had a jokey conversation in the pub about Rahman, Britain would need to have been a Stasi state with informers everywhere for the police to learn of it. Because they joked on Twitter all Rahman needed to do was search for mentions of his name and flam up a case.

The Director of Public Prosecutions has offered some sensible guidelines on Twitter prosecutions. If the Web is not to remain such a boon for narks and sneak, we need a wider cultural change, however. We could do with understanding that we have moved from a world where it was difficult to find discreditable information about someone. Once, if you learned that the police had investigated a man who was applying for a job, the nugget of information assumed a vast significance. Because information was so hard to find, you could assume that the one discreditable fact you had in your possession was the tip of an iceberg, to use the cliché. Now so much information is recorded, we ought to be surprised if we can’t find something discreditable on the Web, or information that can be twisted to make someone look discreditable.

When we realise how far technology has changed we will understand that we ought to be as worried about the Lutfur Rahmans and all the other bullies and grasses who manipulate gullible journalists and police officers as the supposed villains they “unmask”.