I've been using Eric Vitiello's relationship schema to add more information to my FOAF file. At the moment this is experimental so you have to include both the foaf:knows element and the relevant relationship element to ensure that all the cool FOAF tools can understand your social network. One neat tool that does read the relationship schema (plus a whole lot more) is Morten Frederiksen's FoaF explorer which is a fun way to explore foaf-space without any special plugins.