Using partners that have existing payment systems would be one option, but many of the most obvious partners have become direct or quasi-competitors to Facebook over recent years, including both Amazon and Apple. Once a user’s first transaction is completed, the rest is straightforward, but it’s that first transaction that’s going to be the hard part. Crack that, and Facebook should quickly become a major player in e-commerce, adding yet another arrow to its quiver and opening up a whole new set of revenue streams.