Groupon Inc, the largest online coupon company, may be worth billions of dollars less than its recent initial public offering filing suggests, e-commerce analyst Sucharita Mulpuru estimated on Wednesday.

Mulpuru, of Forrester Research, warned investors against investing in the IPO and said future share offerings from social-networking companies like Facebook will be wiser investments.

There is no rational math that could possibly get anyone to the valuation Groupon thinks it deserves, the analyst wrote in an open letter to potential investors. This IPO game isn't about finding value, it's about finding a greater fool who actually believes the valuation is true. Trust me, you will be the fool.

A Groupon spokeswoman declined to comment on Mulpuru's letter.

Groupon filed to sell shares to the public last week, saying it hoped to raise at least $750 million. The document disclosed surging sales, but also high marketing costs that have so far left the company losing money.

Despite being currently unprofitable, the IPO may value Groupon at $20 billion or more.

Forrester's Mulpuru questioned that valuation on Wednesday, arguing that Groupon may be worth closer to $2 billion.

Groupon generated more than $700 million in revenue last year, but Mulpuru said $615 million of that came from acquisitions, heavy marketing and expansion into new markets.

That part of the so-called growth story is not sustainable, she wrote.

Excluding that, Groupon generated $96 million in 2010 revenue from truly organic growth, she noted.

That still means Groupon grew by 223 percent last year. But at 20 times revenue, the company would be worth roughly $2 billion, much less than the numbers out there now, Mulpuru wrote.

Still, Jeffrey Grau, principal analyst at eMarketer, countered that the daily-deal and online coupon sectors may grow so much in the future that Groupon can thrive.

Although there are risks involved in daily deals and challenges for these companies, local commerce is a huge untapped market, Grau said. It's very competitive and not all companies will make it, but there's a lot of money to be made.

There are trillions of dollars spent by consumers at local businesses such as restaurants, retail stores and beauty shops and only a fraction of these transactions currently happen online, he noted.

(Reporting by Alistair Barr; Editing by Gary Hill)