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 Friday, the viral trend had transformed into a fundraising phenomenon, generating a £2 million (HK$25.6 million) windfall for Cancer Research UK.
The #nomakeupselfies campaign raised the money in just 48 hours, with hundreds of thousands of donations from Facebook, Instagram and Twitter users sharing photos of themselves without make-up and then 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.
The charity'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. This has taken off like crazy." The trend appears to have started 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 actress whose looks - apparently affected by plastic surgery - were 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 campaign in which Cancer Research UK had tried 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 on Thursday and Friday and said those users would have donated £180,000.
Eccles said the charity had tried to push fundraising using selfies before but that nothing had taken off like this campaign, which has prompted donations from around the world.
"I'm surprised it's still going, but it shows the strength of feeling behind it," he said.
The unprompted surge in donations mirrors the phenomenon seen after the death of 30-year-old Claire Squires during the London Marathon in 2012.
More than £1 million was donated via the fundraising page she had used on JustGiving to raise money for the Samaritans before her run.
Eccles admitted the link between selfies and cancer seemed tenuous. "It is difficult to try to find that link, but it has done that job because they are thinking about fundraising," he said.
However, the potency of the campaign seems to lie in the combination of the endless vanity of the selfie, the social obligation of being nominated or encouraged by a friend - and the emotional pressure to fundraise to help prevent the disease.
By Friday the trend had moved on, with the variously ingenious and disturbing #sockselfie, #sellotapeselfie and #manandmakeup emerging, populated largely by men, some of whom have encouraged donations for men's cancer charities.