The scene: Shoreditch House (the new east London members’ club) last weekend; the cast: middle-youthy, middle-class media types; the conversation: “Nice Facebook picture, love the Warhol vibe. How many friends have you got? Only 30? Never mind, it’ll pick up. You know, Ricky Gervais is on it.”

Both parties consider this a substitute for actual conversation and edge away to find other “friends”. Everybody at the party is either on it, thinking about joining or is an official refusenik. Even five months ago this wouldn’t have been the case, but the Facebook phenomenon is greasing the wheels of middle-class social life faster than Nigella’s goose fat.

The student community website set up by a Harvard student called Mark Zuckerberg in 2004 is now the 13th most used search engine in the world. I became one of Facebook’s 30m registered users six weeks ago when I overheard my teenage daughter saying to one of her friends, “So I told him to stalk me on Facebook”.

Up until then I thought I was keeping up with the generation Z-ers. I have a pink iPod Nano, earrings from Topshop and I’ve seen every episode of Skins, but I felt my Facebook ignorance was the sign that I had finally tipped into middle age. Within minutes I was at my laptop registering. Three clicks later I was perusing five pages of my daughter’s gorgeous pouting friends. I felt an indecent thrill at being down with the kids.

Next morning I turned on my computer to find three messages from people wanting to be my “friend”. Two of them I had worked with briefly and one was my sister. By the end of the week I had 14 friends and was checking in at least twice a day. It turned out that Facebook wasn’t just for teens, it was for fortysomething mothers and everyone else. In my office of 50 or so twenty and thirtysomethings, everyone is on Facebook, all of them having joined in the past three months. The most popular person is a 28-year-old old-Etonian comedy producer with a record 350 friends.

The tipping point: the moment when something goes from being niche to being mainstream has arrived. This morning I got an e-mail from a schoolgate mother asking to be my friend. Yummy mummies have found the network, which may explain why Hugh Fearnley-Whittingstall has no fewer than five appreciation groups.

So what’s the appeal? On a basic level it is a return to the playground where kids define themselves by their friends and which gang they are in. Facebook started as a college site; until autumn 2006 you could join only by invitation and the software still encourages you to define yourself according to your education. As my daughter puts it, “the randoms” are kept at bay because while someone can look you up and see who your friends are, they can’t access your profile unless you have approved them as a friend. (Not that such niceties stopped the Oxford University proctors who have been allegedly using the site as a way of keeping track of students.)

There is something quaintly old-fashioned about the codified social exhanges of Facebook. It’s the cyber equivalent of the Emily Post: asking to be someone’s “friend” is the 21st-century equivalent of leaving your visiting card at someone’s house. If they find you socially acceptable they return the call, otherwise you are ignored without the unpleasantness of the snub direct.

In my experience, though, there are not many Lady Bracknells on Facebook imperiously denying access. When I get a friend request from someone I don’t know, I am too craven to ignore them, on the basis that I would be mortified if it happened to me. Many people are quite brazen about claiming “Facebook friendship” with people they have never met. One of my new friends says it is the new e-mail. I have never set eyes on him but I know he reads The New Yorker, misses Australia and wants a basset hound for Christmas. A quick skim of his photos shows that he looks as lissom in Speedos as he does in a tux. I’m not sure I need to meet him now. I’ve seen the trailer, do I need to see the movie?

The question of how much information is too much is a tricky one. On the one hand Facebook is the modern equivalent of the “hatch, match, dispatch” page of The Times, a place where you can announce life’s milestones to your peer group. “It’s a great place to see your friends’ baby pictures hours after the birth,” says a thirtysomething colleague.

But that can have unintended results. A friend of mine joined because “all the other Whitehall policy wonks were on it”. Two weeks later she was horrified to find a weird ex among the “friends” of a friend. Instantly she took all the personal details off her profile, changing her relationship status from the little red heart – which means going steady – to single.

Minutes later her mobile phone was humming with real-world friends offering to comfort her through her “break-up”. She said: “I was amazed people would think that I would announce something so serious on Facebook first.”

Another thirtysomething woman in a long-term relationship was mortified when her partner refused to confirm on her profile that they were “in a relationship”. He sees it as an invasion of privacy. She thinks it shows a lack of commitment.

I’m too old, I think, to have any Facebook skeletons; my exes are struggling to work their mobile phones, but for some people it is something of a Pandora’s box. I have even heard cases of “Facebook suicide”, when a person “kills off” their online profile because too many ghosts of the past have been dug up.

I’m sure there are Facebook affairs, although I wonder if such lovers ever actually meet. A survey by uSwitch.com says hardcore broadband users spend more than 10 hours a week on virtual networking sites, the equivalent of 24 days a year, which is more than the 22 days spent socialising with real people. Who needs real friends or the exchange of bodily fluids if you can be made to feel popular and hip each time you log on? Who needs flowers when you can get electronic cupcakes?

My habit is under control, but only just. It has taken far too long to write this piece because I stop and check my profile every half hour on the off chance that I’ve got a new friend. One person who won’t be writing on my online wall is my daughter, who has rejected all offers of “friendship”. She is even muttering about defecting to the wrinkly-proof Bebo or the “hardcore” MySpace: “Your Facebook thing is beyond sad, mum, but if you enjoy it I can’t stop you.”

I’m not sure I’d want to stay on a social network if my mother was a member.

Rachel Johnson is away

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