Logan Paul had a message for his six million Twitter followers: He was “all in” on a new cryptocurrency called Dink Doink.

According to the project’s creator, Dink Doink investors would receive shares of a cartoon character, entitling them to a portion of the proceeds if the googly-eyed figure ever appeared in a TV show or movie. Last June, Mr. Paul, a 27-year-old boxer and social-media influencer, praised Dink Doink on Twitter and in a public Telegram chat, before endorsing it again on his podcast, “Impaulsive.”

But by mid-July, the price of Dink Doink had plummeted to a fraction of a cent, and Mr. Paul was facing an online backlash. In his endorsements, he had failed to mention some relevant information: He and the project’s creator were friends, and they had come up with the idea for the cryptocurrency together. He had also received a large allocation of Dink Doink coins when it launched.

“I don’t know what went absurdly wrong,” Mr. Paul said in an interview. “That’s the project from hell, and I just wiped my hands of that.”

The collapse in crypto prices this month has renewed scrutiny of the celebrity marketers who sell virtual currencies to the masses. Over the last year, the actor Matt Damon and the comedian Larry David have starred in high-profile TV commercials for crypto platforms, trumpeting digital assets as an unmissable moneymaking opportunity. Those ads drew criticism from crypto skeptics, but they were tied to mainstream companies with hundreds of millions of dollars in revenue.

A far seedier form of crypto promotion has flourished on social media, rife with undisclosed conflicts of interest and exaggerated claims about skyrocketing profits. Celebrity influencers like Kim Kardashian and Floyd Mayweather have made millions of dollars endorsing specific and often dubious crypto investments, urging fans to buy obscure coins that quickly crashed in value, or shilling little-known collections of nonfungible tokens, the unique digital files known as NFTs.

In some cases, promoters like Mr. Paul have admitted that they failed to disclose personal or financial ties to projects advertised on their feeds, a potential violation of federal marketing regulations. And even before the crypto market’s recent downturn, a series of these influencer-backed ventures had crashed spectacularly, hurting amateur traders and prompting lawsuits that could force some celebrities to compensate investors for their losses.

“You have this shameless profiteering from celebrities and others, who aren’t at all disinterested or impartial,” said John Reed Stark, a former chief of the internet enforcement branch at the Securities and Exchange Commission. “There is a lot of potential for harm.”

Crypto entrepreneurs hire influencers to push up the value of their digital currencies, hoping to ignite the sort of online hype that briefly turned Dogecoin, a joke currency based on a meme, into one of the most valuable crypto investments.

Some promoters are not well known outside crypto circles but have large followings on social media, where they broadcast market tips, interspersed with sponsored content. Others are major celebrities like Ms. Kardashian, who is facing a lawsuit from investors over her marketing of an obscure cryptocurrency called EthereumMax.

The amounts paid to crypto promoters can be astronomical. An NFT project called Hive Investments has been recruiting influencers, offering payments as large as $400,000, according to a presentation reviewed by The New York Times.

Jordan Belfort, the former stockbroker whose memoir inspired the 2013 movie “The Wolf of Wall Street,” was once offered $250,000 to change his Twitter profile picture to an NFT. Mr. Belfort, who recently rebranded himself as a crypto guru, turned down the offer.

“We don’t want to be a part of things that basically exist purely to separate people from their money,” said Matt Hirschberg, Mr. Belfort’s business partner. “I’ve had people offer us guarantees of up to at least $10 million just to get involved.”

Crypto promotion occupies a legal gray area. Under federal law, people marketing securities are required to publicly disclose payments for promotions. In 2018, Mr. Mayweather paid more than $600,000 to settle S.E.C. charges that he had failed to properly disclose his compensation for marketing initial coin offerings, the crypto equivalent of an initial public offering on Wall Street. But the rule he broke applies only to securities, like stock in a company, and it is unclear which crypto products meet that legal standard.

Crypto promoters could also run afoul of the Federal Trade Commission’s rules, which require marketers of all kinds to disclose when they have a financial stake in the projects they endorse.

“Companies and the social media influencers of the world view this as the Wild West,” said David Klein, a lawyer in New York who specializes in marketing rules. “The old-world laws still apply, and you have to follow the guidelines. Otherwise, the regulators will come calling.”

Even celebrities who disclose crypto payments have found themselves in legal trouble. Last summer, Ms. Kardashian endorsed EthereumMax in an Instagram post with a brief disclaimer at the bottom: “#ad.” Few crypto insiders had hear