Groupon Inc subscribers have more than doubled to 115 million so far this year, a source with knowledge of the situation told Reuters, a positive sign for the daily deal site's much-anticipated 2011 IPO.

The company, which is vying with LivingSocial and a growing number of new entrants in the online deals business, had 50.58 million subscribers at the end of 2010. That jumped 64 percent to 83.1 million at the end of the first quarter.

Since March 31, the number of subscribers has climbed about 38 percent to 115 million, according to the source, who declined to be identified because the information has not been made public.

Most of the recent growth has come organically rather than through acquisitions, the person added, noting that Groupon has not bought many companies lately.

Groupon, started by Chicago music major Andrew Mason in late 2008, has filed to raise $750 million in an IPO this year

Revenue rose to $713 million last year from $30 million in 2009. In the first quarter of this year revenue topped $644 million.

There are few growth opportunities on the scale of companies like Groupon, said Lou Kerner, vice president in equity research at Wedbush Securities covering social media and e-commerce. That's really what a lot of investors are seeking today.

But critics have questioned whether its growth can be sustained given increasing competition with Google, and Facebook, as well as a growing number of deals sites.

Groupon spent $208 million on marketing during the first quarter of this year, to increase the number of subscribers and to generate revenue growth, up from $4 million in the same period a year earlier.

Groupon had a net loss attributable to common stockholders of $146 million in the first quarter of this year.

A slower pace of subscriber growth may help improve Groupon's bottom line, according to David Sinsky of Yipit, which aggregates daily deals and tracks the industry.

Groupon generates cash from existing subscribers as some of them buy its deals, while the company spends money attracting new subscribers, he explained.

The key question is how much money are they spending per new subscriber and how much revenue are they generating per existing subscriber? Sinsky said. If you assume that revenue per subscriber has held flat and costs for new subscribers were also the same, then profitability may look better.

Others have cast doubt on some of the metrics Groupon has provided - including one of its own invention, Adjusted CSOI, or consolidated segment operating income.

Groupon will drop Adjusted CSOI from its IPO filing when the filing is amended early next week, All Things D reported on Friday. A spokesman for the company declined to comment.

Adjusted CSOI excludes online marketing expenses, stock-based compensation costs and acquisition-related items.

In the first quarter of 2011, Groupon reported a $117 million operating loss, but adjusted CSOI was almost $82 million. That's because nearly $180 million of online marketing spending and more than $18 million in stock-based compensation was excluded.

Groupon has been criticized for its use of adjusted CSOI because some analysts and investors question whether Groupon will be able to cut marketing spending in the future.

(Reporting by Alistair Barr; Editing by Phil Berlowitz, Bernard Orr; editing by Carol Bishopric)