An excellent note by Terri Bradford of Kansas Fed.
It shows how social networks are being used in payment systems/commerce:
This article begins with a glimpse of the extent to which social networks have been adopted. Next, it describes the diversity of commerce arising among social networks and the payment methods that support it. The article then describes ways in which financial institutions are using social networks to provide banking services and how consumer attitudes may drive opportunities to offer person-to-person (P2P) payments. Finally, the article concludes by setting out some of the potential risks of these various interactions.