A screenshot from @UNICEF_uk’s Twitter stream. (Twitter)

In the wake of the ALS Ice Bucket Challenge — a meme/fundraiser/cultural tornado that ruled the social Web for weeks — every nonprofit with an Internet connection and a dream has lusted over an ice bucket moment of its own.

There were, initially, attempts at a “Rice Bucket Challenge” (against hunger, in India) and a “Rubble Bucket Challenge” (on behalf of bomb victims in Gaza). The actor Orlando Jones attempted a “Bullet Bucket Challenge” to draw attention to Ferguson. Later, as the meme grew old — as memes invariably do — marketers began peddling less literal takes on the charitable social media stunt: the Camel Toe Challenge, for cervical cancer awareness; the Taco Beer Challenge, to raise money for an abortion-rights group; the Eat Pie for HI challenge, to benefit kids who suffer from hyperinsulinism.

You have probably not heard of too many of these hashtags, because — especially compared to the Ice Bucket Challenge — they straight-up didn’t work. And yet, another knockoff has just now entered the fray: It’s called #wakeupcall, and it’s the well-meaning (if slightly desperate) brainchild of UNICEF UK, the British branch of the United Nations Children’s Fund.

In this new and appropriately random challenge, the subject snaps a selfie when he wakes up, nominates three people to do the same, and donates £5, or $8, to UNICEF’s Syria appeal. Since @UNICEF_UK launched the campaign on Monday, the hashtag has been tweeted roughly 15,000 times. The participation of a number of British celebrities, including Hugh Grant, Tom Hiddleston, Stephen Fry and Jemima Khan, the journalist who kicked off the campaign, has led some to prophesize that #wakeupcall will be the next #icebucketchallenge, generating millions of dollars and attention-raising tweets for a critical cause.

Alas, while the hashtag’s done pretty well thus far, the chances of ALS-level virality are slim. Traffic on the tag has dropped precipitously since Hiddleston shared his selfie two days ago, and — per Twitter analytics tool Dataminr — the majority of the tweets have stayed confined to Britain. That’s not necessarily the campaign’s fault. The chances that anything will be “the next ice bucket challenge” are remote.

“Resist the pressure to mimic the Ice Bucket Challenge,” Nathaniel Ward, a fundraiser for the Heritage Foundation, wrote on his personal blog in August. “There’s no plan you can copy … [and] you can’t recreate the conditions for success.”

Those conditions, one nonprofit expert told The Wall Street Journal, include a web of seasonal, subjective and otherwise serendipitous factors: It was summer, it was easy, “it touched people,” it was fun. Later research suggested the hashtag was driven primarily by young, sporty men, a demographic most ALS fundraisers wouldn’t think to target.

Think of the meme in purely evolutionary terms. (Evolution is, after all, where the word meme comes from.) The social Web is an unforgiving, Darwinian place. The only ideas that thrive are the ones deemed reproducible by large, disparate cascades of people. Organic hashtags that nonprofits jump onto later — a la #icebucketchallenge — have grow up in this environment. They’ve proven that they can survive in the wild.

Successors like #wakeupcall, on the other hand, have been handed down from on high, strategized by high-powered consultants and pushed by celebrities. They follow all the purported best practices of the Ice Bucket Challenge: They’re silly, they’re youthful, they explicitly nominate other people to join in. And yet, just because these campaigns follow the ice bucket playbook doesn’t mean they’ll see ice bucket-level success.

You can’t make fetch happen; it has to happen on its own.