Facebook is starting to monetize its massive membership, and not just in ads. Games on FB generate revenue for both FB and the games developers. To wit, FB users buy Facebook credits, which are used to purchase virtual goods in these games. If users buy such credits in an iOS app, FB (and the FB developers) may have to pay 30% to Apple, based on iOS rules. I'm not sure how this is happening right now, but the potential is there for Apple to demand this 30%, which is potentially massive. Zuckerberg is heading this off by making sure web-based access is a viable if not superior FB experience for its users.