(Bloomberg) -- Even after one of the worst stock-market debuts in more than a decade, two 30-year-old founders of SmileDirectClub Inc. still have plenty to smile about.

Alexander Fenkell and Jordan Katzman are both billionaires, as is Katzman’s father, Chief Executive Officer David Katzman, 59, following an initial public offering of shares in the maker of dental aligners.

SmileDirectClub plunged 28% to $16.67 Thursday, making it the first U.S. firm since at least 2008 to raise more than $1 billion and price its IPO above range, yet fall in its inaugural trading day. The shares, offered at $23, rebounded Friday, climbing 11% to $18.55 at 12:08 p.m. in New York.

It has been a busy September for the public markets and those it enriches.

WeWork’s Adam Neumann has seen the value of his stake drop by more than $10 billion, at least on paper, as Wall Street tempered expectations for the office-sharing company amid lingering corporate-governance concerns. On Tuesday, exercise-bike maker Peloton Interactive Inc. said it planned to offer 40 million shares to raise more than $1 billion, which would give co-founder John Foley a $450 million fortune.

And on Friday, shares of cybersecurity-software developer CloudFlare Inc. surged 27% to $19 apiece in the minutes after their New York Stock Exchange debut. That boosted the stakes of CEO Matthew Prince and fellow co-founder and operating chief Michelle Zatlyn to $717 million and $277 million, respectively.

Read more: WeWork plows ahead on IPO and seeks to rein in CEO’s control

SmileDirectClub’s young founders join the growing ranks of young self-made billionaires like Facebook Inc.’s Mark Zuckerberg, 35, and Snap Inc.’s Evan Spiegel, 29.

Despite WeWork’s flagging fortunes, Neumann, 40, will still be a billionaire three times over if the company goes public with a market capitalization of $15 billion, the lowest current projected valuation.

--With assistance from Ben Stupples.

To contact the reporters on this story: Sophie Alexander in New York at salexander82@bloomberg.net;Tom Maloney in New York at tmaloney38@bloomberg.net

To contact the editors responsible for this story: Pierre Paulden at ppaulden@bloomberg.net, Peter Eichenbaum, Steven Crabill

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