Groupon Inc, which more than doubled subscribers this year to 115 million, plans to abandon the use of a controversial financial measure it once touted as a good indicator of performance, two sources with knowledge of the situation said.

The No. 1 daily deals website, which now dwarfs closest rival LivingSocial's membership base, caved to pressure from investors and will stop referring to a metric called Adjusted Consolidated Segment Operating Income (ACSOI) that excludes marketing costs, one of the sources told Reuters.

Groupon, speeding toward one of 2011's most highly anticipated IPOs, plans to file amended S-1 IPO documents next week giving an update of its performance, both sources said.

The phenomenal pace of growth will be unveiled in that filing and likely give its IPO a boost, despite fears about its need to spend heavily to lure new users and worries of another dotcom bubble brewing reminiscent of the late 1990s.

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

Founded by Northwestern music major Andrew Mason in 2008, Groupon filed to raise $750 million in an IPO this year. In April, a source said Groupon could raise $1 billion, valuing it at $15 billion to $20 billion.

Social media firms like LinkedIn Corp have had spectacular debuts, stoking interest for offerings by the likes of Facebook and Twitter. But doubt is growing on Wall Street about whether the buzz surrounding the new Web generation is justified, with the hype recalling the atmosphere prior to the dotcom collapse of 2001.

That caution may have led some to question Groupon's use of ACSOI, which excludes not just online marketing expenses but also stock-based compensation and acquisition-related items.

Other investors question whether Groupon will be able to cut marketing spending in future.

In the first quarter of 2011, Groupon reported a $117 million operating loss, but ACSOI was almost $82 million. That's because some $180 million of online marketing spending -- plus more than $18 million of stock-based employee compensation -- had been stripped out.

Tech blog All Things D first reported that Groupon would drop all references to ACSOI in future IPO filings. The news comes after reports the U.S. Securities and Exchange Commission was taking a closer look at Groupon's IPO -- and ACSOI.

A spokesman for the company declined to comment.

DOUBTS GROWING

Groupon offers discounts on everything from dining to sky-diving excursions. The group in its name refers to the fact that many deals are activated only when a certain number of people sign up. Discounts often run from 50 to 70 percent.

It had 50.58 million subscribers at end-2010. That jumped 64 percent to 83.1 million at the end of the first quarter. Since March 31, that number of subscribers climbed about 38 percent to 115 million -- several times LivingSocial's.

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

Most of the recent growth came organically rather than through acquisitions, a second source added, noting that Groupon has not bought many companies lately.

Despite a sizzling pace of growth, some critics remain wary about piling into a business -- essentially a coupon service -- that can be easily replicated both by startups and existing Web powerhouses. Google has already begun such a service.

And while it shares the spotlight trained on social media companies such as Zynga, it needs resources others don't: a huge sales staff to enlist merchants and handle customer service.

Groupon has spent a lot to lure subscribers and generate revenue growth. It shelled out $208 million on marketing alone during the first quarter of this year, up from $4 million in the same period a year earlier.

The key question is how much money are they spending per new subscriber, and how much revenue are they generating per existing subscriber? asked David Sinsky of Yipit, which aggregates daily deals and tracks the industry.

If you assume that revenue per subscriber has held flat and costs for new subscribers were also the same, then profitability may look better.

(Additional reporting by Edwin Chan; Editing by Phil Berlowitz, Bernard Orr)

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