Facebook chief operating officer Sheryl Sandberg says that the social networking company doesn't plan to charge users... yet.Top executives, particularly savvy veterans heading up a company that's all about growth, are very careful with their words -- take my word for it. So it's fair to pay close attention to Sandberg's exact words to BusinessWeek: "we are not planning on charging a basic fee for our basic services."

Basic services? A slip of the tongue? In the very next sentence she repeated, "We're not going to charge for our basic services."

Clearly, Sandberg is hedging because she knows Facebook is going to start charging users for something in the near future, and doesn't want to damage her credibility. The guessing here is that Facebook isn't going to start charging a subscription fee but will, however, for some of the apps its users download. That's why it has signed up online billing service Zuora, the brainchild of former Salesforce.com executive Tien Tzuo.

There are currently some 50,000 apps on Facebook, and plenty of developers hoping their efforts will get them a few bucks from a portion of Facebook's 200 million users. Facebook would cut itself a substantial portion of that and go a long way towards proving that they have a viable business.

The question, though, is whether Facebook users would ever pay for an app on the service. I know I wouldn't... probably. How about you?