The difference is subtle. Currently, social companies like Facebook offer login/identify services, and different companies like PayPal offer payment. But do consumers trust Facebook for anything other than social (like shopping)?

Conversion is everything for online merchants. Identifying customers early in the shopping experience allows better marketing, better engagement, and higher conversion. Plus, to be able to pay again in 1-click is huge (no additional login, no entering credit cards). So ultimately it means more $$ for merchants.