(REUTERS) -- Zynga Inc's shares fell 16 percent on Wednesday, amid heavy volumes, after the social gaming company warned of a slowdown in bookings, indicating paying players may be spending less than expected.

A lackluster fourth-quarter showing -- Zynga's first as a publicly traded company -- and expectations of sequentially slowing bookings in the first half of 2012, may rattle bullish industry watchers who have driven up the company's valuations.

Core game monetization is slowing more rapidly than expected, Macquaries Equities Research wrote in research note.

For 2012, Zynga projected bookings between $1.35 billion and $1.45 billion -- a year-over-year growth of 16 percent to 25 percent. Bookings grew 38 percent to $1.16 billion in 2011.

Zynga uses bookings as a metric to measure the cash it gets upfront when people spend money on virtual items in its games such as tractors, houses or poker chips.

At least three brokerages downgraded the stock.

The owner of the popular Farmville and Cityville games does not disclose the number of its unique payers on Facebook, making it difficult to gauge any possible deceleration trends.

Investors are closely watching how Zynga -- which gets almost all its revenue from Facebook, as it said while filing for its IPO last year -- diversify and make money from games on smartphones and tablets.

Evercore Partners said the company's stock -- trading at 31 times the brokerage's 2013 estimate for earnings before interest taxes depreciation and amortization (EBITDA) -- bears risk, and downgraded it to under weight from equal weight.

The company, which went public in December, trades nearly 67 times forward earnings, compared with the sector average of 12, according to Thomson Reuters data.

Analysts, however, remain convinced that Zynga is well positioned for longer-term growth.

Zynga was one of the key developers that turned Facebook from a relatively passive communications medium to a more active and engaged social platform, Robert W. Baird & Co wrote in a research note, and downgraded the stock.

While growth has slowed for both Facebook and Zynga, long-term secular shifts in content consumption, along with significant growth opportunities on smart devices from Apple and Google are too compelling to ignore.

Barclays Capital also cut its rating on the stock.

On Tuesday, Chief Operating Officer John Schapper said in an interview Zynga's games are designed to gain popularity and make money in the longer term. ... That's why we still have the six-most played games on Facebook.

The company's Words with Friends, a Scrabble-like game, recently hit headlines after actor Alec Baldwin got kicked off an American Airlines flight for playing the game on his iPhone while the plane was parked at the gate.

The gamemaker's stock, which has jumped more than 30 percent since its Dec. 16 debut, fell to $12.11 in early morning trade, making it one of the biggest percentage losers on the Nasdaq.

More than 22 million shares had changed hands by 10.55 ET, almost close to their 10-day average volumes.

(Reporting by Sayantani Ghosh in Bangalore; Editing by Joyjeet Das)