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The post was made at lunchtime on Wednesday by a Mumsnet user called Riven.

The entry, in the site's chat area, was simplicity itself. Riven Vincent told the site's users - her online friends - that she had asked social services to take her severely disabled daughter into care.

"We get 6 hours respite a week," it read. "They have refused a link family, they have refused extra respite. I cant cope."

The entry sparked a hugely supportive reaction from the site's users and snowballed into a major front-page story across the national media inside 24 hours.

Riven Vincent, it transpired, had met the prime minister before the election. He had made a private visit to meet her - after an exchange on Mumsnet.

'National institution'

On Thursday his office said he was "very concerned". David Cameron was writing to her and was getting in touch with her MP.

The incident is yet another illustration of the power of Mumsnet, or rather the site's 600,000 registered users.

It was co-founded 10 years ago by sports journalist Justine Roberts and TV producer Carrie Longton. Today it has 1.25m unique users every month.

Justine is now the Mumsnet chief executive and last year she hosted the then prime minister Gordon Brown at its 10-year anniversary bash. Mr Brown described it as a "national institution".

Political muscle

While Mumsnet has hit the headlines before, most notably in a legal battle with the childcare writer Gina Ford, it is only relatively recently that its members have started flexing their political muscle.

Justine Roberts isn't that surprised by the developments this week. She says interest from politicians started to hot up during the last general election, along with a realisation that social media was an effective way to reach voters.

Both Mr Brown and Mr Cameron did "live chat" sessions with Mumsnet, whose members exhibit a marvellous tendency to "ski off-piste" with their interrogations.

Mr Brown famously fumbled an answer about his favourite biscuit, while the Conservative leader appeared to stop answering questions for several minutes when his laptop broke.

But Mumsnet does much more than quiz politicians. Funded entirely by advertising, it functions as a resource for information on pregnancy and childcare, offers product reviews and even runs a book club. Tonight it is hosting a chat session with the award-winning author Rose Tremain.

Mumsnet users sometimes have a vexed relationship with the media, and in recent weeks the site took the producers of EastEnders to task over a plotline involving a child abduction.

Held to account

"All we did was reflect our members' reaction to the storyline," explains Ms Roberts. "These things are often entirely organic, coming directly from our members rather than directed by us as the site's administrators."

The internet is notoriously capricious, with a site being essential one moment and forgotten the next. But Mumsnet's capacity to hold decision-makers to account shows no sign of fading.

"With Mumsnet," says Ms Roberts, "there's a sense of audience. Facebook is too generalised.

"Riven Vincent's post was shocking, and it caused an immediate reaction. She's known in our community as a caring mother."

But she admits to being surprised at the size of the story. "It is extraordinary how quickly these things can mushroom."