A Slate summary of a recent CNN article (see the source for the CNN link)
In a recent press conference, Facebook CEO Mark Zuckerberg observed that more Facebook users are concerned about the site charging for use than about the privacy of the content they post there. There are hundreds of groups calling for Facebook to remain free of charge or warning that the site will begin charging for access. Facebook employees repeatedly quash these fears by saying they would never charge for content because "putting up a paywall runs counter to the company's mission to make the world more open and connected." Whether you believe that explanation or not, the company also has a financial incentive to keep the site free. Facebook can attract more members without charging for access, and more participants in the network means more eyeballs to attract lucrative advertisers.