Justin Post of Bank of America Merrill Lynch forecast Facebook beating earnings estimates but said the report will hinge on whether ad revenue is accelerating or decelerating.

"We estimate that mobile revenues could reach $90mn in Q3, up from roughly $25mn in 2Q, although PC revenues could be down q/q (some revenue cannibalization)," writes the analyst, in an earnings primer.

At 14% of overall ad revenue, mobile earnings came in at nearly double of Post's forecast and helped to drive strong year-over-year and sequential revenue growth. In the third quarter Facebook's revenue rose 32% from year-ago levels.

As with Apple's ( AAPL) iPhone 5 launch, some analysts worried Facebook may face the challenge of meeting user demand for its products as it rolls out new ways to advertise on the social network.

The bull case on Facebook has long centered on the network's still unquantifiable earnings prospects and its growing popularity, but investors should also take stock of obvious risks to the company's share prices through year-end.

In particular, Facebook faces a lockup expiration, where $1.2 billion of shares -- 700 million shares in total - could hit the stock market in the next three months, noted William Bird of Lazard Capital Markets, in a note prior to earnings.

Meanwhile, economic weakness in Europe and a muddled picture on fees paid by social gaming giant Zynga ( ZNGA) were obvious risks headed into the quarter. In October, Zynga badly missed earnings and cut guidance for the second consecutive quarter.

Facebook's fee revenue for the third quarter came in $176 million, a 13% a 9% decline sequentially from the second quarter of 2012, indicating an impact from Zynga's woes.

While Zynga's struggles forced fee-based revenue, however, Facebook made strong progress in generating new mobile ad revenue.

For more on Facebook, see why the company's valuation shouldn't be a trick question on Wall Street. Also see why a venture capital investor says to think long-term on Facebook after its weak post-IPO performance.

-- Written by Antoine Gara in New York

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