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6 Biggest Facebook IPO Surprises

Facebook's S-1 threw up some surprises when it was filed after market close on Wednesday, as the social networking giant gears up for its much-hyped$5 billion IPO.

From Mark Zuckerberg's security detail to a robust mobile payments forecast and a heavier-than-expected reliance on Zynga (ZNGA), prospective investors were given some revealing insights into Facebook's world

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1. Slowing Growth

Despite boasting over 845 million users across its network, Facebook says in its so-called S-1 document that it expects its active user growth rate to decline over time as it achieves higher market penetration and increased competition. These signs are already starting to show.

While the number of users in Brazil and India continues to climb -- representing increases of 268% and 132%, respectively -- growth in the U.S. has slowed to 16%. Facebook's ability to increase its user base is critical to revenue growth by influencing the number of ads that the site shows, the value of those ads and the number of payment transactions that occur.

"If we are unable to maintain and increase our user base and user engagement, our revenue, financial results, and future growth potential may be adversely affected," the S-1 said.

2. Mobile Revenue

One of the most surprising elements of Facebook's S-1 is that, despite the company's heavy reliance on mobile platforms, it does not make any money from its mobile products.

Of the $3.71 billion in revenue Facebook generated in 2011, no "meaningful" revenue was generated from its mobile platforms, the company said.

Facebook had more than 425 million users accessing Facebook through its various mobile products (iPhone, Android, iPad, and BlackBerry apps) in December 2011. The company said that it expects mobile growth will actually surpass the growth rate of overall users (845 million) as it continues to focus on developing mobile products. "We do not currently directly generate any meaningful revenue from the use of Facebook mobile products, and our ability to do so successfully is unproven," the filing said.

3. Greater-Than-Expected Reliance on Zynga

FarmVille maker Zynga generated 12% of Facebook's revenue in 2011 through payment processing from the sale of virtual goods as well as advertising purchased through the social gaming firm. If Zynga chooses not to feature its games on Facebook's platform or if Facebook fails to maintain a solid relationship with Zynga, its business could be severely hurt.

Zynga users must use Facebook Credits -- the social network's virtual currency -- to pay for goods on the site. Facebook then keeps a 30% cut of goods purchased from Zynga's games.