Andreas Bernstrom, CEO of Rebtel, the world's largest mobile VoIP company after Skype, said that Facebook could add hundreds of millions in revenue by introducing a mobile calling service.

According to Bernstrom, looking at a typical Facebook user profile, approximately 15% of a users' friends reside in a foreign country, and with 900 million users, this translates into approximately 135 million potential customers. With typical Skype conversion rates falling around 6%, for Facebook this would mean the potential to immediately have upwards of 8 million credit cards on file.

Applying this to Skype's current ARPU of approximately $8 per user a month, Facebook could expect to generate revenues of up to $64 million dollars a month, or very close to $800 million in revenue annually by providing a service similar to Skype Out to handle international calls.

"Everyone has been critical of Facebook's inability to monetize on mobile, but the social network has a massive opportunity to leverage its vast user base through international calling," said Bernstrom. "Given its attractive social graph, even if Facebook didn't reach the same conversion rates as Skype, the huge call volume associated with even a fraction of its users could result in dramatic revenues for the company."

Many believe that Facebook's strategy on mobile is critical to its long-term value. The social network, which recently bought Instagram for $1 billion, also launched its own photo sharing app Camera in May, in what appeared to be a move to increase its photo sharing app market share and further build its mobile product offerings.

In a revised S-1 filing with the Securities and Exchange Commission Facebook stated: "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."

Rebtel believes that integrating a mobile VoIP calling service will immediately alleviate recent investor concerns that Facebook's revenue per user has been in decline and an increasing amount of its users have shifted from desktop to mobile. The company also thinks that operators already realize a viable threat, as the social networking giant, with its more than 488 million mobile users, already surpasses Vodafone, the second largest operator in the world with 44O million users.

Bernstrom continued: "Facebook has great opportunity to build an app for calling functionality in much the same way they have one for messaging and photos. So next time you would comment on a friends' photo who lives abroad, or when you send them a message, a pop up or ad could appear such as "Do you want to call this user now?" Not only does this generate a very large revenue potential with the possibility of increased conversions, but it also allows Facebook to start building a giant database of credit cards, which will be hugely important for future cross selling opportunities."

Research firm Strand Consult recently reported that Facebook is already starving mobile operators of its traditional SMS cash cow, "transporting more mobile traffic, number of messages and time spent online than the world's largest operator (China Mobile)." Cutting into another huge cash cow of voice would be yet another blow for the telecom industry, which many research firms have already noted faces stiff competition from OTT services like Skype, Viber and Rebtel, and continues to struggle to reduce customer churn.