Google Social Graph API

Google announced the release of the Social Graph API with a focus on public connections between people on the Web.

Here's how it works: we crawl the Web to find publicly declared relationships between people's accounts, just like Google crawls the Web for links between pages. But instead of returning links to HTML documents, the API returns JSON data structures representing the social relationships we discovered from all the XFN and FOAF. When a user signs up for your app, you can use the API to remind them who they've said they're friends with on other sites and ask them if they want to be friends on your new site.

Links:
Social Graph API homepage
Developer documentation
Social Graph API Group
XHTML Friends Network (XFN),
Friend of a Friend (FOAF)