Facebook has allowed applications on its platform to charge their users for in-app purchases for a while now: virtual tractors in FarmVille and so on. Now the social network is introducing recurring subscriptions into the mix – a feature that could be used by streaming music services. So far, the companies working with Facebook’s subscription billing are games companies like Zynga and Kixeye, where players who subscribe get exclusive content and extra virtual currency every month. However, TechCrunch has a screenshot mocked up by Facebook showing a user paying Spotify and MOG through its system too. However, if Facebook wants companies like this to bill customers through its system, it may have to drop its traditional 30% revenue share.