A psychotherapist friend was explaining why she had forbidden her 12-year-old daughter from joining Facebook. It had driven several of her patients, around the same age as her daughter, to the verge of mental breakdown. But surely these girls were unusually fragile: if not Facebook, wouldn't there have been some other catalyst? Maybe, she said, but few young egos are strong enough to deal with this stuff.

I thought she was being alarmist and somewhat old-fashioned. Our generation merely utilises the internet: our children have it hardwired into their synapses. It is their medium, just as ours was television: our parents fretted similiarly - and impotently - about its new-fangled consequences.

You are supposed to be 18 to join Facebook. But you can lie about your age; no one checks. When my sons signed up I thought it sweet when they befriended their aunties and old babysitters, sent virtual pina-coladas to far-flung godparents. Then I realised they're all on there! The entire lower school, the whole prepubescent lot of them, “poking”, posting preening party pictures and telling each other “u are soooo pretty!!!”.

My friend's warning was amplified this week when a trial with implications for the future of social networking opened in Los Angeles. Megan Meier, 13, was befriended on MySpace by a boy called Josh Evans who flirted and flattered and told her she was “sexi”. When he dumped Megan abruptly, saying the world would be a better place without her, she went up to her bedroom and hanged herself with a belt. It transpired “Josh” was a 49-year-old mother called Lori Drew who, it is alleged, believed Megan was bitching about her own daughter online. Drew is charged with conspiracy and accessing computers without authorisation, not murder. But the prosecution case is that Drew “fully intended to hurt and prey on Megan's psyche” through MySpace.

It is an outlandish and extreme story. Yet what struck me was how Megan's mother's reacted when her daughter came to her sobbing about Josh's cruelty. She told her she shouldn't get into silly arguments and shouldn't have been on her computer anyway. Clearly, she believed her daughter was wasting real emotions on something which was “unreal”, since it took place online. Many parents, I guess, would have been equally dismissive.

It is a quandary we have not yet addressed, despite Britons spending more time online (an average of 14 hours a week) than any other European nation and with half of us now members of social networking sites: can the virtual world cause real pain? Facebook seems so harmlessly middle-class, like an endless online evening drinks party. For us sad, solitary home-working types it is a simulacrum of cheering human contact.

But my friend suggested I look at Facebook with a 12-year-old's eyes. She pointed out the popular “honesty box” application where you ask a question - “What do you really think of me?” etc - which then anyone can answer anonymously. Like a ouija board, evil yet so tantalising. My inner pre-teen came out in a terrified sweat.

Besides, said the psychotherapist, it is the ordinary stuff which devastates her patients, the photos of a sleepover to which you weren't invited, your best friend ignoring you and chatting on someone else's “wall”. And everyone will know, by how many friends you have, whether you're a big, fat loser. It's not even proper bullying, just crude kidult passive- aggression. But, boy, does it hurt.

Even so, her patients cannot stop themselves logging in. They have to look. And so the mean-girl snubs, the whispering behind hands, follow them home and upstairs into lonely bedrooms.

We think as adults we are tougher, that something as remote and notional as a chat room cannot hurt us. Indeed, it is a blast, a liberation, when talking online to say what you really mean for once, to make mischief, to dispense with uptight British niceness, or even assume the guise of an atavar, a pumped-up, better-hung version of our own weedy workaday self.

In the glow of our screens, safely at home, we think our egos are armour-plated. But there is no protection as we step on to the ten-lane superhighway of a billion heartless strangers. It can smart like hell, that withering rebuke from someone you'll never meet. A friend, who frequents a jolly and supportive parenting website, was devastated when another mother posted “I hope your child fails the 11-plus”, particularly when she discovered the woman was a neighbour, who'd always harboured a secret grudge.

We are a fighty nation at present, itching for a scrap like a railway station drunk. Perhaps, because we feel impotent in the face of huge economic forces, we lash out at more accessible targets - Ross and Brand, Haringey social workers, the judges on Strictly Come Dancing. And our anger spews onto the BBC's Have Your Say messageboards, blogs and newspaper websites.

This morning I was forwarded a letter from a reader who berated me about something I wrote last month, with the use of two C-words and sundry other curses. From the handwriting - and by the simple fact it came by snail-mail - I could tell it was written by an elderly person. It had no address and was signed “No Nonsense Norm”. Poor Norm, I thought, with his thin notelet, shaky pen and his probably painful walk to the postbox. With a computer he could have enjoyed the same secret thrill of hate in an instant, and free.

Most journalists, me included, find the honesty box below our words bracing: in the democracy of the web, why should we claim a monopoly on thought? Others, though, find the comments too confidence-destroying to read. (If you blog about us, do we not bleed?) Although few, like the hack hero in Tim Dowling's hilarious novel The Giles Wareing Haters' Club actually track down and confront their tormentors.

Maybe future generations will learn to deal with the strong and confusing emotions engendered by the virtual world. Friends with older teens say that they log into social sites before breakfast, know the etiquette, how seriously to take it, where to complain. And later my friend rings to say her daughter just 'fessed up to having a secret Facebook account. What can we do? Not much. Online we're elderly residents of a new world, just like Norm.

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