Rubhub is an XFN relationship lookup engine for determining the relationships between people who author personal websites. As such it is the first application to make use of XFN.
I introduced XFN data in a previous post. In essence it uses the rel attribute of the <a> tag to denote personal relationships between the two websites at either end of the link, for instance defining the linkee as a friend, colleague and neighbour.
Rubhub crawls blogrolls and other lists of ‘recommended’ websites and looks for XFN data to display personal relationships between site owners. Here is Clagnut’s social network as defined by XFN.