SAN FRANCISCO — Floyd Mayweather, perhaps the greatest boxer of his generation, is not shy about using social media to display the wealth that his years of prize fighting have won him. On Facebook, you can find videos of Mr. Mayweather draped in diamond chains. Want to see him with blocks of $100 bills taped to his torso? There's that, too.
Recently, Mr. Mayweather has shown his appreciation for a new kind of money. In September, he told his 13.5 million followers on Facebook not once but twice that they should buy a new virtual currency known as the Centra token.
"Get yours before they sell out," he wrote above a picture of himself admiring the many boxing title belts he had been awarded over the years. "I got mine and as usual I'm going to win big with this one!"
Mr. Mayweather is among the many celebrities who have recently endorsed an initial coin offering, the name for a hot but loosely regulated new method of fund-raising in which entrepreneurs sell their own virtual currencies to investors around the world.
The boxer's endorsement of Centra, along with a similar endorsement from the popular rapper DJ Khaled, lent a patina of credibility to a project that has ended up with more than a few problems, including a chief executive who does not appear to have been a real person and a shaky, fast-shifting business plan.
Thanks in part to the endorsements, in just a few weeks Centra's founders raised over $30 million from investors around the world. They finished their fund-raising this month, just before a grand jury indicted two of the three co-founders on perjury charges stemming from a drunken-driving case.
Centra was one of the 270 or so I.C.O.s that have raised more than $3.2 billion this year, a 3,000 percent jump from last year's total, according to data from Tokendata.io, which tracks coin offerings. Investors have been willing to pay real money for these virtual tokens because they hope their value will go up as fast as the price of Bitcoin, the best-known digital currency, has in recent months.
Celebrities have helped stoke the I.C.O. boom. The actor Jamie Foxx, the socialite Paris Hilton and the soccer player Luis Suarez, for example, have all promoted new virtual currencies to their sizable followings on social media in recent months, offering legitimacy and attention to coin offerings that might have otherwise gone unnoticed.
Mr. Mayweather, who has promoted three different tokens — Centra, Stox and Hubiits — has even taken to calling himself Crypto Mayweather in social media posts, a play on his better-known nickname, Money Mayweather.
But the story of Centra illustrates that beneath the signs of mainstream acceptance, coin offerings still exist in a legal gray zone with few checks on the ambitions of young entrepreneurs.
"It's undeniable that a celebrity endorsement brings a new audience into the world of crypto currencies," said Peter Van Valkenburgh, the director of research at Coin Center, a nonprofit that advocates for Bitcoin and related technology. "But I'm not certain that celebrity endorsements are doing a good job of bringing attention to the legitimate projects."