Social sign-in has become a powerful force for marketers and consumers, validating the notion of federated identity in consumer-facing contexts. (Ironic that consumerization of IT is successfully tackling even the single sign-on problem that has bedeviled IT, showing how identity for the top line of the business can overcome resistance in ways that business-to-employee scenarios typically can't.)
But not all consumer-facing federated SSO is social. When I was with PayPal, our team worked on the underpinnings of what eventually turned into Log In with PayPal, which is strictly about federated identity flows for commercial purposes. And today Amazon has come out with Login with Amazon, a powerful statement of Amazon-as-identity-provider. They've been testing this with their own web properties Zappos and Woot; now they're enabling third-party merchants and other sites to use Amazon for authentication of people who already have active Amazon accounts, along with learning a few selected user attributes: name, email, and optionally the zip code of the default shipping addresses. No huge social graphs here, just data that partner eCommerce sites need to function (and make money).