NEW YORK (

TheStreet

) --

Zynga

(ZNGA) - Get Report

shares dropped to their lowest level

since the company's December IPO

after a research report questioned the social gaming firm's reliance on a small number of customers for the majority of its revenue.

Zynga shares fell as low as $8.64 in Friday morning trading following a report from Macquarie Group analyst Ben Schachter who launched coverage of the company with a neutral rating.

Schachter said that while he is bullish on the video game industry's transition from packaged goods to online, he's wary about Zynga's overreliance on just a handful of users for most of the company's revenue.

Only about 2.2% of Zynga's 150 million users pay to play its games, he said. And of these paying customers, some 680,000 account for over 70% of Zynga's $800 million annual revenue. Known as "whales," these customers fork over several dollars for virtual items in such as a barn or chicken for Zynga's

FarmVille

title.

But these dollars add up -- Schachter estimates the top 20% of paying players spend over $1,100 per year on Zynga games.

An overdependence on certain users to spend big bucks is just one issue that Zynga investors are grappling with.

The company's growth potential is another worry, Sterne Agee analyst Arvind Bhatia said in a recent note to investors.

"While we believe in the potential for social games, we think Zynga's growth is slowing even faster than what is obvious at first," he wrote.

Zynga may be having trouble attracting new users, as well.

Although Zynga converts about 90% of its players from an existing game to the next, only 10% of gamers are new,

based on a September survey

from video game networking site Raptr.

Zynga is also battling new competition on

Facebook

, where it has dominated as the leading player up until now.

The Sims Social, a game from

Electronic Arts

(ERTS)

launched on Facebook in August and quickly became the second most popular game on the platform.

--Written by Olivia Oran in New York.

>To follow the writer on Twitter, go to

http://twitter.com/Ozoran

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