Tens of thousands of women shared social media pictures of themselves under #nomakeupselfies before Cancer Research UK piggybacked to raise funds.
It started with a row at the Oscars, featured the current obsession with "selfies" and rapidly clogged up legions of Facebook streams.
Tens of thousands of women, egged on by their friends, shared pictures of themselves without make-up – to raise awareness of breast cancer.
By last Friday the viral trend had transformed into a fund-raising phenomenon, generating a £2-million windfall for Cancer Research UK.
The #nomakeupselfies campaign raised the money in just 48 hours, the charity said.
This happened through hundreds of thousands of donations from Facebook, Instagram and Twitter users sharing pictures of themselves without make-up, and nominating a friend to do the same.
Cancer Research UK said it had not initiated this particular campaign, but was alerted to the #nomakeupselfies trend on Tuesday and began to ask users to add a donation request and text code to their posts. Since then, the money has flooded into the British charity.
Cancer Research UK's head of social media Aaron Eccles said: "We're over the moon. When we do a social media campaign we want to engage as many people as possible, and this has taken off like crazy."
The idea itself appears to have begun last week when American crime author Laura Lippman tweeted a picture of herself without make-up in support of Kim Novak, the 81-year-old actor whose looks had been criticised at the Oscars.
The theme was picked up by celebrities and fans of Lippman, before spreading more widely.
But for all the unexpected financial success, some commentators question whether it is appropriate or relevant to link the "bravery" of appearing without make-up to the very different challenge of fighting cancer, triggering fierce debates online.
Blogger Yomi Adegoke said: "Thinly veiling vanity as philanthropy more than irks ... the pretence these images are for anything other than an onslaught of 'natural beauty' acclamations, coupled with pats on the back for 'fighting the cause' makes the no make-up selfie mania even harder to stomach."
At some point, all the criticism prompted people to remember and then resurrect a previous failed social media campaign, when Cancer Research UK had attempted unsuccessfully to use selfies to raise awareness of breast cancer.
Twitter said the hashtag started to gather steam on Tuesday and peaked on Thursday, with 83 000 mentions since Wednesday alone. Instagram saw 59 000 posts in 24 hours from Thursday to Friday, and estimated those users would have donated £180 000.
Eccles said the charity had tried to push fund-raising using selfies before. But nothing had taken off like this campaign, which has prompted donations from around the world. – © Guardian News & Media 2014