• Billionaire investors Stanley Druckenmiller and Bill Ackman were early backers of Coupang.
• The South Korean e-commerce giant nearly doubled from its initial public offering price shortly after Thursday's midday open.
• Coupang raised $4.6 billion in its IPO, the largest one in the U.S. so far this year.

South Korean e-commerce giant Coupang, which soared in its Wall Street debut, got early backing from a couple high-profile investors: Stanley Druckenmiller and Bill Ackman.

Coupang, dubbed the Amazon of South Korea, nearly doubled from its initial public offering price of $35 per share shortly after Thursday's midday open on the New York Stock Exchange.

The stock later pared those gains and closed up almost 41% at $49.25 per share, giving Coupang a market capitalization of $84.5 billion.

Druckenmiller, the billionaire CEO of the Duquesne Family Office, has been a longtime, pre-IPO investor in the Seoul-based company, Kevin Warsh, an advisor to Druckenmiller, told CNBC's Becky Quick. Warsh, a former Federal Reserve governor, joined the board of Coupang in 2019. Warsh owns a total 280,662 shares of Coupang, according to a filing with the Securities and Exchange Commission.

Ackman, the billionaire who runs the Pershing Square Capital Management hedge fund, invested personally in Coupang, a source close to the situation told CNBC. It's unclear when that investment was made. But a Reuters report in 2014 mentions Ackman as an investor.

Coupang raised $4.6 billion in its IPO, the largest one in the U.S. so far this year. The company sold 130 million shares at $35 apiece Wednesday evening, above its target range of $32 to $34.

The company was founded in 2010 by Bom Kim, who continues to serve as CEO. Other investors include Masayoshi Son's SoftBank Group.

"When we talk about Coupang, what it is, it is Amazon but it is Amazon with a UPS attached to it, with DoorDash, with Instacart, with a little dash of Netflix and that is all integrated on this technology platform with an extreme degree of customer centricity," said Lydia Jett, an investment partner at SoftBank's Vision Fund and a member of Coupang's board since 2018.

SoftBank's Vision Fund owns about a third of Coupang, having invested billions of dollars into the company. In an interview on CNBC's "Squawk Alley," Jett said it didn't take long to realize Kim is a top-notch founder worthy of backing.

"When I met Bom and spent three days in Seoul with him, I was blown away by the level of customer understanding and the customer centricity of his company, the innovation that was taking place," Jett said. "It was clear to me that this company was doing something radically different from its competition and the customers were responding," she added. "You can see that in the numbers of the company."

Coupang's total revenue was $12 billion in 2020, up nearly 91% from the previous year. It registered an operating loss of $527.7 million in 2020 — down 18% from 2019 and down nearly 50% from 2018.

The company was ranked No. 2 on the CNBC Disruptor 50 list last year.