All aboard the IPO train! Next stop, Chicago.

Groupon, the rapidly-growing digital coupon service, filed an S-1 registration statement with the Securities and Exchange Commission on Thursday indicating its intention to sell shares in an initial public offering. The Chicago-based startup is just the latest internet company to go public this year, following Demand Media, LinkedIn, and Zipcar, among others.

The rush of companies that have gone public — or are expected to do so — has fueled a vigorous debate about whether the internet sector is entering a bubble akin to the one that burst a decade ago. (Of course, depending on whom you talk to, we’ve been in a “tech bubble” every year for the last five years, dating back to Google’s then-shocking $1.65 billion purchase of YouTube, so your mileage on “bubble” may vary.)

Groupon, which currently loses money, plans to raise as much as $750 million in the offering, which will be underwritten by Wall Street giants Morgan Stanley, Goldman Sachs and Credit Suisse. The IPO could result in a valuation for Groupon of over $15 billion, according to numerous media reports, with some estimates reaching a staggering $25 billion, as Bloomberg reported in March.

But wait. Didn’t Groupon raise nearly $1 billion just a few months ago? Where did all that money go? The company’s registration statement contains the answer, as noted by AllThingsD’s Peter Kafka.

Since December of last year, Groupon paid out an eye-popping $810 million to early investors, including founder and CEO Andrew Mason. (See chart above for details on one of the sale rounds). Chicago-based investor Eric Lefkofsky walked with over $300 million, while the Samwer brothers — perhaps Germany’s most high-profile startup investors — pulled a cool $170 million, according to the filing.

Last year Groupon had sales of $644 million, but lost money, to the tune of $450 million. The company claims 83 million subscribers.

In a letter to potential investors, Mason said the company is focused on long-term growth, even at the expense of short-term gains.

“In the past, we’ve made investments in growth that turned a healthy, forecasted quarterly profit into a sizable loss,” Mason wrote. “When we see opportunities to invest in long-term growth, expect that we will pursue them regardless of certain short-term consequences.”

Groupon’s rise has been nothing short of meteoric. Last year, Groupon’s revenues grew by over two-thousand percent, earning it the moniker, “the fastest growing company ever,” by Forbes. That may help explain why Groupon was confident enough to turn down a reported $6 billion buyout offer from Google, a move that left many observers shaking their heads in amazement. If Groupon’s IPO is successful, the company will be more than vindicated.

Mason is known for his colorful antics, and his letter to prospective investors did not disappoint. “We are unusual and we like it that way,” he wrote, adding, “Life is too short to be a boring company.”

Mason cautioned investors that “the path to success will have twists and turns, moments of brilliance and other moments of sheer stupidity.”

Early reaction was mixed on Wall Street, but skeptics of the new internet gold rush were quick to pounce on Groupon’s valuation.

“I think it’s egregiously priced,” Paul Meeks, an analyst at Capstone Investments, said on CNBC Thursday afternoon. “Even though revenue is growing very quickly, it is profitless prosperity.”

Meeks said the low barriers to entry in the daily deal space mean that a Groupon copycat — and there are already several dozen of them — could knock the company off its perch as the leader in the space.

In the company’s IPO risk factors, which provide a glimpse of possible threats to the business, Groupon acknowledged the threat posed by imitators, as well as entrenched companies like Google and Facebook, which are eying the deal space hungrily.

“Our competitors may offer deals that are similar to the deals we offer or that achieve greater market acceptance than the deals we offer,” the company wrote. “This could attract subscribers away from our websites and applications, reduce our market share and adversely impact our gross margin.

“In addition, we are dependent on some of our existing or potential competitors, including Facebook, Google and Microsoft, for banner advertisements and other marketing initiatives to acquire new subscribers,” the company continued. “Our ability to utilize their platforms to acquire new subscribers may be adversely affected if they choose to compete more directly with us.”

For now, Groupon leads in the deal space, but it knows that the competition is coming, which may have added urgency to its plans to go public.

So far this year, internet offerings have seen huge one-day pops. Web auto rental company ZipCar raised $174 million and watched its stock price close up 60 percent in its first day of trading. Demand Media, which went public in January, enjoyed a 33 percent first-day pop.

Facebook, which is valued at more than $50 billion on private markets, is expected to IPO next year,