We generate substantially all of our revenue and players through the Facebook platform and expect to continue to do so for the foreseeable future, Zynga wrote in its IPO prospectus.

Technically, the admission is called a risk factor. But since Zynga, the wildly popular maker of mobile and social games such as Mafia Wars and FarmVille, generates about 95 percent of its revenue through Facebook, the worry for investors is that its relationship with Mark Zuckerberg's social network is less a risk factor than a business model.

And how Zynga ultimately performs as a public company -- it is aiming to raise $925 million at a $9 billion valuation when it begins trading on the Nasdaq on Friday -- will depend in large part on its ability to break free from Facebook. Or at least its ability to convince investors that it can do so.

So far, however, the skeptics remain unconvinced. At Zynga's IPO roadshow luncheon in San Francisco on Monday, investors spent most of the question and answer time with Zynga executives asking about Facebook.

Any time you have such a large reliance on a single company, you have to be concerned, said Dan Niles, chief investment officer of AlphaOne Capital Partners, who didn't attend the luncheon but watched one of Zynga's presentations over the Internet.

From an investment perspective, ignoring the fact that all but 5 percent of Zynga's $828 million in revenue in the first nine months of this year came from Facebook could be detrimental.

Zynga conceded that point in its IPO prospectus, noting that, any deterioration in our relationship with Facebook would harm our business and adversely affect the value of our Class A common stock.

Facebook takes a 30 percent cut of the revenue Zynga derives from the social network, which features more than 222 million monthly active Zynga users, according to the data tracking website AppData. Zynga itself makes most of its money from less than 3 percent of its players, who buy virtual items like trucks and poker chips.

Being so dependent on one company clearly poses risks to Zynga's growth potential. If Facebook's user growth slows, for instance, Zynga's growth is likely to slow as well. Or, in an extreme case, if Facebook suddenly decided to banish games, it could harm Zynga's entire business.

Zynga is also beholden to Facebook in other ways. According to a regulatory filing on July 18, the company has to publish some of its games exclusively on Facebook before other platforms.

What's worse, Zynga may have botched one main attempt it has thus far made at trying to break away from Facebook.

When the company unveiled its new online platform Zynga Direct during a rare media event at its San Francisco headquarters in October, it was billed as a way for Zynga to deal directly with its consumers without an intermediary.

But when players visited Zynga's website to sign up for a user name, called a Z Tag, they were told to first install the Zynga app on Facebook, giving the impression that it was being more closely integrated with the world's largest social network instead of being weaned off of it.

A Zynga spokesman on Wednesday declined to comment on the company's IPO.

FACEBOOK FLIPSIDE

The counter argument is that Zynga's reliance on the platform may attract investors looking to bet on Facebook's growth. With Facebook's IPO at least several months away, there currently are not many ways to gain exposure to Facebook on the stock market.

Ahead of Facebook's IPO, Zynga is the closest proxy investors have, said Robert W. Baird & Co analyst Colin Sebastian. As of today, Zynga is highly dependent on Facebook and could bring in investors who are looking to find ways to gain exposure to social media.

Akram Yosri, managing partner of 3i Capital Group, attended Zynga's roadshow presentation in New York and said he was satisfied with how management responded to questions about Facebook and how Zynga can grow in partnership with the social network.

They didn't dodge the question, said Yosri, whose firm has $1.4 billion in assets under management. As long as it's a working relationship, it's a plus for Zynga because Facebook is going to be there a long time and has a proven business model.

Zynga also gathers lots of data on its millions of users, more than half of whom are female, which marketers could find attractive.

CHINA AND BEYOND

Still, Zynga faces a long road to a less Facebook-dependent future. According to regulatory filings, Zynga's contract with Facebook doesn't come up for review until 2015. This gives it three years to find new revenue sources outside the social network such as moving into new markets like Asia and making more games for mobile devices.

In July, Zynga entered mainland China's games market for the first time, partnering with Chinese platform Tencent for a local version of the game CityVille.

Zynga has at least until that time to expand its presence in Asia and it is trying to do that aggressively in mobile, said Steve Sorrano, an equity analyst at Calvert Investment Management, which has $12 billion of assets under management.

Sorrano added, however, that investors might find Zynga too risky to bet on while it is building out its business in these new areas since it is unclear whether the company can deliver a high enough or sustainable return on capital investment.

That's a relative unknown for a young company in an industry that is developing this rapidly. This raises risks for going in(to the stock) that early, Sorrano said.

Zynga's total expenses rose 115 percent to $747.9 million in the first nine months of the year, a sign that its international ambitions are adding to costs.

With regard to mobile, while games such as Words With Friends have become hits, its roughly 13 million mobile users are dwarfed by the hundreds of millions of users who play it on Facebook.

While Zynga was one of the earliest game makers on the Facebook platform, it lacks that first mover advantage on mobile. For instance, Disney released a mobile game in September called Where's my Water that is ranked ahead of some Zynga titles in Apple's App store.

And investors said that Zynga may already be losing market share on Facebook itself, as video game companies such as Electronic Arts make large acquisitions to compete with it. Indeed, AlphaOne's Niles pointed to EA's The Sims Social game, which has 28 million monthly active users, as a successful example of encroachment by another video game company on Zynga's turf.

Over time, however, there is hope the Zynga can break free from Facebook with services like Zynga Direct. Though that service still has no date for when it will launch or which games will be available, Sterne Agee analyst Arvind Bhatia said that it should help reduce Zynga's platform risk somewhat.

(Reporting By Liana B. Baker in New York, additional reporting by Alistair Barr; Editing by Peter Lauria and Steve Orlofsky)

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