Although there are a few different standards for highlighting links like these, most social networking sites, blog software and hosting services support one or more. They're relying on site builders to highlight such links by using special markup. Google uses this information to build a graph of all sites and their links to one another.

Rather than request the API from each site individually (Twitter followers, MySpace friends, etc.), Google provides a single RESTful API to send a simple query in one shot. While there are limits to this data (Facebook info is currently not available in this widget), the information we're getting is real and the friend information we're getting isn't a simple number but a list of live URLs that Google has vetted for us.