One of Facebook's (FB) biggest growth engines may also pose the biggest risk to its stock.

There is no doubt that growth in Internet-enabled devices such as smartphones and tablets has played a vital part in getting Facebook past the 1 billion users mark. However, compared to desktops, the monetization opportunities on mobile platforms are lower.

This case can be compared to Pandora's (P) case where the Internet radio company has struggled with its mobile monetization and thus has not been able to register profits until now.

Compared to the monetization of close to $63 per 1,000 listener hours for desktop, Pandora's mobile monetization stood at around $20 per 1,000 listener hours in 2011. The case could be similar for Facebook and was the reason why its overall revenue per 1000 page views dropped dramatically from 47 cents in 2009 to around 29 cents today in 2011.

The problem is simple. Facebook's users are increasingly accessing the social network on mobile devices where there is not much real estate to put ads. Additionally, Facebook does not serve audio ads unlike Pandora, and thus is completely reliant on the visual ads.

Even though the problem is straightforward, the solution is not. Any attempt to significantly increase the ads on mobile platform will affect the user experience negatively due to limited real estate on the mobile screens. Keeping this in mind, we forecast a very low growth in revenue per 1,000 page views. We expect this figure to increase from close to 29 cents in 2011 to 36 cents by the end of our forecast period.

However, there is a chance that Facebook's efforts to effectively monetize the mobile platform fail and the growth in international markets further puts downward pressure on the monetization levels. If these factors actually push down the monetization to 20 cents per 1,000 page views by the end of our forecast period, there could be 40% downside to our price estimate for Facebook.

We have already incorporated lofty expectations for mobile growth in our forecasts for total user base and page views per user. We forecast Facebook's total user base to double to 2 billion by the end of our forecast period and expect the page views per user to continue to grow as well.

Our price estimate for Facebook stands at $28, implying a premium of 40% to the market price.