Google I/O 2008: URLs are People Too - Using the Social Graph API to Build a Social Web
Using email addresses to identify people has a problem - email addresses can be used to send, not receive. With the rise of blogs and social networks, millions of people are using URLs to refer to themselves and others. The Social Graph API indexes these sites and their connections, enabling this web-wide distributed social network to be used to make your sites better. Learn how XFN and FOAF express connections, how we index them, and how OpenID combines with The Social Graph API to help connect people on the web to your applications, and save your users from re-entering their friends over and over again.
Kevin Marks, Brad Fitzpatrick (Google)