I would never open my boyfriend's post. He wouldn't let me, for one thing. I'm not even allowed to open his copy of Paws – the Battersea Dogs Home newsletter – until he gets home. And I'd never open his emails either, but I don't have the same privacy taboo about emails that have already been opened. I have picked over the cobbles of past boyfriends' email trails. It starts with a simple over-the-shoulder visual eavesdrop. When you walk up behind somebody, and they're in the middle of an e-conversation, they hunch. Everybody does this, colleagues do it. It doesn't mean they're cheating on you, it just means it's personal. But this tension, the email that isn't a betrayal but is nevertheless private, is a micro-version of the surveillance debate – "Why do you want privacy, if not to commit crime?"

Back to this hypothetical boyfriend who, if I'm honest, was actually a real boyfriend. He hunches when I walk past. So I want to see what he's writing. I can't, he's closed his browser. It does not feel evil to go into his inbox afterwards, although I would strongly dis-recommend that initial breach of couple-protocol. Of course, it's an email to a woman. What are the odds? Fifty-fifty, unless he's a mathematician, or a nitwit. Say it's someone you know (it was), it's probably an ex (it was). Say it's someone you don't know, then it's a strange woman. This is a mug's game; now I'm checking all the time. My nearly ex and his ex chat intermittently. Nothing incriminating happens, but the easy familiarity is annoying. And I'm reading everything I can get away with now, so naturally I catch stuff I'd rather not have seen. It's physiologically compelling (high risk, sweaty palms) and intellectually boring, like reading a Dan Brown novel. Unlike a Dan Brown novel, nothing happens, except I can't stop. I never got caught doing it. But we did split up, for maybe 1,000 reasons, some of which were: well, most obviously, this is an act of war. It also creates confusion – you're having your regular relationship with your regular boyfriend, and a secret, antagonistic relationship with the boyfriend as expressed in his correspondence. Those are two different people, not least because one of them doesn't know you're there.

Women snoop a lot more than men – a joint study by the LSE and Nottingham Trent University found that 14% of wives read their husbands emails, and 10% checked their browsing history (for men, those figures are 8% and 7%, respectively). I know what you're going to say, you'll say, "That's because men look at porn all the time. Women are just looking for evidence of porn, and maybe if they spent more time looking at their own porn instead of spying on their husband's, these figures might be reversed." That's what I'd say.

Still, we often talk about the nefarious things men get up to on the internet. You hear about porn addicts. You hear about men who lie to teenage girls on Bebo, men who sit on Chatroulette all day. The things you hear about men make them sound so profoundly primitive, you wonder how they hit the space bar without an opposable thumb.

There's no doubt the internet creates a new territory of misdemeanour, but not all of it particularly male. When people talk about predatory men, or naive and/or bullying teenagers, they miss the major UK demographic, the one in which we outstrip internet usage anywhere else in the world, which is among housewives. That definition is pretty loose, these days; you don't have to be married, and you're allowed to have a job. It just means women of a certain age. Any given woman who, 10 years ago, would have been out binge drinking: women like me, and possibly you.

UK housewives spend 47% of their leisure time online (according to a study by global market information group TNS), which is higher than the Chinese national average (overall theirs is the highest in the world). Our national average is 28%. Some of this is entrepreneurial (almost half of all UK housewives make some money online – one in 20 "mousewives" makes over £200 a week), but a lot of it is pointless messing about.

And because we're women, and many of us have children, this messing about is billed as an incredibly positive, cooperative force. Indeed, Mumsnet has become the byword for mothers on the internet, as if all we do is have warm, helpful conversations. It's true that Mumsnet has a lot of users (20 million monthly page impressions), and everything its founder, Justine Roberts, says about it makes perfect sense: "It's become a very handy, convenient and efficient replacement for real-life communities. People just don't have time for leisurely conversations over the garden fence any more. Women and parents in general don't have time to have a lot of social engagements in the traditional sense and Mumsnet fills that void."

There is an unspoken point, though, isn't there? Not having time for social engagements is the same as being lonely. Virtual conversations aren't really the same as real ones: they're so conditional, so easy to pick up and drop, they don't carry the weight of a concrete connection in the world. It's a community and yet the succour isn't real, the responsibilities users feel towards one another are quixotic, evanescent. It's suspended between life and a computer game.

Contrary to popular presentation, Mumsnet is not the only site women visit. There are acres of girly chat. Not very much chat-traffic is criminal or exploitative, but check out the Facebook groups for a flavour of how unpleasant some of the supposedly mumsy stuff is. There's a proliferation of vigilante rage directed at child abusers: "jamie bulger's killers should never have been released!"; "i bet i can find a billion people who are against jon venables and r thompson!!!!"; "Don't forget about Maddie"; "Justice for Baby P". The numbers of signatories are enormous – sure, at over 37,000 names, the Venables/Thompson page loses a bet with itself about finding a billion. But 37,000…

There are 245 groups calling for the death/life sentence/dismemberment of Vanessa George, the nursery worker who took pornographic photographs of her charges and exchanged them with a man and woman she'd met on Facebook. It is taken as a paradox of George's case that, when you track her Facebook history, before she got involved in online paedophilia, she would sign up to groups like Action Against Abuse. In fact, I don't think it's paradoxical. There's something zealous and savage about the anti-paedophile rhetoric on Facebook that doesn't seem to have anything to do with children, or sex with children – it seems to be about whipping yourself to a pitch of fury that is in itself arousing, it's like rage-porn. The comparison is instructive: like regular porn, this self-generated anger might be elemental, but previous to the internet, it was something you might glance past in the Sun; 37,000 people wouldn't be devoting their leisure time to hating paedophiles.

Which brings us back to Vanessa George. The first time she and her co-defendants met was in the courtroom. In her first police interview, George tells how she went from Facebook buddies with the man to sharing images of paedophilia. "I was, like, What would you do for me, if I done that for you? You'd have to put a ring on my finger to make me do things like that." The fact she already had a husband of 20 years standing is the least bizarre element of this self-presentation – as just another young woman, looking for love ever after, who'll do anything for a ring on her finger.

Apparently, members of the investigative team privately doubted all three when they claimed to have met on Facebook, thinking it too much of a coincidence that people with such depraved tastes would just chance upon one another on a mainstream networking site. It certainly pushes the boundaries of credulity that there are paedophiles ambling the corridors of Facebook, waiting to meet one another to scale up their perversion. But if you look at the Facebook application these three met on, Are YOU Interested?, you would have no trouble believing it to be just heaving with incredibly lonely, violently angry people, just sitting there, ripe for a toxic relationship with other incredibly lonely, violently angry people.

It's a Honeymoon Killers cliché: if you want to find a lonely person, look at the lonelyhearts ads. But the modern version is so blunt, the disappointment and vulnerability so poorly disguised, it's a con-artist's fairground. There's certainly a small-time criminal element, recognising this lonely constituency and tapping them for cash. Joann Wood, 53, caught the tabloid imagination last year due to the fact that she's a lesbian and focused her efforts on lesbian and gay sites. Wood lied to suit every occasion, starting with "I love you" and ending with anything from cancer to an expertise in the gold trade. She made about £100k before she was discovered (and sentenced to four-and-a-half years in prison). But the internet is just the facilitator to Wood; 15 years ago, she would no doubt have found some pre-internet strategy, like working in Nationwide, befriending pensioners with big savings.

The Jihad Jane case in America, by contrast, could happen only now, with this timely confluence of global communication and a terrorist movement whose targets are international. Jihad Jane, whose real name is Colleen LaRose, was arrested last year over her plan "to do something, somehow, to help suffering Muslims": it stretched, in the end, to a conspiracy to murder the Swedish cartoonist Lars Vilks. The 46-year-old has been accused of conspiring to provide material support to terrorists, and kill a person in a foreign country. She is also suspected of having trawled the internet looking for other women with US passports who could more easily go about the skirmishes of Jihad undetected. Her boyfriend of five years had no idea of these activities. "She was a good-hearted person. She pretty much stayed around the house," Kurt Gorman told the press. She was active on the site revolutionmuslim.com, and this is not a place you'd stumble into. What led her there is unknown, but behind the sudden veil-wearing and talk about eternal bliss, this looks like a sad story about grief. In 2005, following the death of her father, LaRose tried to commit suicide. Obviously the causal links are complicated, but she wasn't trying to kill cartoonists before then. If LaRose had conceived an irrational hatred against a neighbour, this would have been a containable affair. But there's an Alice in Wonderland effect on the internet, where a person taken out of his or her context can take on epic proportions in an unfamiliar landscape, usually not in a good way. When physical space is collapsed, people can find themselves a long way from home.

The main point is Morrissey's: the devil will find work for idle hands. There's nothing idler than people on the internet, wanting nothing in particular, just wanting to be nearer the centre of things.