Wired on Friday: Like any busy, anonymous city, the most salubrious corners of the internet are just one wrong turn away from the must salacious. That's the way it is on Craigslist, the smash US classified and personals website, where advertisements for used sofas are just a few clicks from lonely hearts and the semi-explicit requests for company that fill a section called "casual encounters".

But this city story isn't really about Craigslist. You'd see the same mix in the small ads section of any metropolitan paper. This is about the difference between the presumed anonymity of the city and the far more fragile privacy the internet offers. And how it only takes one prankster to cause the veil of immodesty in the most personal of personals to fall, for good.

Last month, on the local Seattle Craigslist site, an advertisement appeared, apparently offering a catalogue of unusually generous services from a mysterious woman for a temporary assignation. The post said she was looking for someone who "was arrogant, self-centred, nasty, egotistic, sadistic".

It was a false advertisement - as many of the most fantasy-inspiring advertisements in personal columns have always been, one suspects. But this fake was different. It was posted by Jason Fortuny, a male Seattle blogger.

He took all of the replies, and reposted them on a public web page.

The advertisement received more than 178 replies. The responses variously contained full names, e-mail addresses, 145 photographs and, in some cases, personal phone numbers. Fortuny left them all intact and online for his friends to laugh at - and for the search engines to scan and store.

The reverberations were instant, and escalated for days. Fortuny already had a reputation as a internet prankster, so the list gained an immediate audience. And this kind of privacy train-wreck quickly attracts high numbers of online rubberneckers, so the victims' data was rapidly linked to and viewed far and wide.

To date, Fortuny's public posting has received 681 replies, and presumably hundreds and thousands of hits.

It was only a (short) matter of time before the faces and names in the replies began to be recognised. One explicit petitioner had sent mail from his Microsoft work address. Another turned out to be the husband of a friend of Fortuny. Others in the hit list discovered their exposure when less inhibited bystanders used the information to mail or telephone them the news.

The friend's marriage was, perhaps irretrievably, damaged. The Microsoft man was fired for using his work account. And Fortuny, whose own details were easy to find online, began to receive the first physical threats against his person.

There are many questions to ask about an episode like this, but perhaps the compelling issue for the technologically minded is - why doesn't this happen more often?

There's little way of assuring an advertisement isn't fake. Internet users appear eager to hand over personal details when asked. Pranksters will be pranksters, no matter the real damage to real lives, and we all have the tools to place information in the hands of whoever wants to seek it out.

That last factor is what makes this a more potentially frequent (and damaging) occurrence than before the existence of sites like Craigslist, which are no more or less at risk from exposures than their offline, newspaper versions.

Perhaps it doesn't happen because we are a little nicer as a species than we give ourselves credit for.

Or maybe this is just the first of many - a site called Craigslist-Perverts.org has emerged to collect future examples. Or perhaps there's just more money in holding such individuals to private ransom than illicit fun in public exposure.

There are laws that could apply to Fortuny's action, in Washington state, although it's unlikely he will be prosecuted. He's certainly done something socially irresponsible, but if we can agree that the effects of such an act should be limited, perhaps the biggest question for all of us is where do we place those limits?

What is the choke point for preventing private information from progressing, in acts like this, as well as larger corporate events such as the leaking of AOL search data earlier this year?

Do we all have some responsibility not to offer information to strangers online that we "wouldn't want to appear on the front page of a newspaper", as the warning has it? Shall we decide to crush and re-engineer the internet to disallow the degree of free speech it provides? Is that even possible?

Or do we adapt? Do we start defending our privacy more vigorously, with laws, practices and technology?

And for the rest - the information we want to share, and expect to hear about others - do we need to develop a thicker skin?

Danny O'Brien is activism co-ordinator of the Electronic Frontier Foundation