May be Great for Blogs?
XFN™ (XHTML Friends Network) is a simple way to represent human relationships using hyperlinks. In recent years, blogs and blogrolls have become the fastest growing area of the Web. XFN enables web authors to indicate their relationship(s) to the people in their blogrolls simply by adding a 'rel' attribute to their <a href> tags, e.g.:
<a href="http://jeff.example.org" rel="friend met">...
The Friend of a Friend (FOAF) project.
The Friend of a Friend (FOAF) project is creating a Web of machine-readable pages describing people, the links between them and the things they create and do.