Full disclosure up front - I work at Janrain, which provides social login technology for websites.
We're seeing very strong interest in using social login on retail and eCommerce sites from our customers. The reason? The marginal/incremental value of a registration taking place is much higher than with other site types, since a registration is often tied to a checkout process. Forrester estimates that retailers experience 23% cart abandonment rates at checkout, because shoppers don't like creating new accounts the traditional way, filling out long forms, or remembering passwords. We think social login solves each of these challenges.
Regarding some of the feedback in this thread, most of the social networks and email providers store an email address for users that they can share with sites when using social login. Facebook, Google, Yahoo!, LinkedIn, PayPal and Windows Live, for example, all provide a verified email address.
Confusion about which identity provider a user previously chose is easily remedied. This blog post discusses how a user's preferred identity can be remembered on future visits to eliminate any confusion - http://janrain.com/blog/how-leverage-social-login-part-2-social-media-best-practices-series/.
Regarding security, obviously if a Facebook user has chosen their dog's name as their password, then they are at risk. But online users are getting smarter about choosing better passwords for high-value sites which they visit with frequency. And companies like Facebook and Google have employed entire security teams devoted to account and password security - it is much more difficult to hack the social networks and email providers than other consumer sites across the web.
Some of our customers have elected to employ features such as second-factor authentication for users who register via social login, which is a pragmatic option on sensitive transactional websites (e.g. financial services).