Now if you think this is just some random thing, this inserts values to the rel attribute of an anchor tag of the link you are linking to. Choosing any of these can trigger it one or any combination of these rel attribute values.

• friendship (pick at most one)
  • contact
    Someone you know how to get in touch with. Often symmetric.
  • acquaintance
    Someone who you have exchanged greetings and not much (if any) more — maybe a short conversation or two. Often symmetric.
  • friend
    Someone you are a friend to. A compatriot, buddy, home(boy|girl) that you know. Often symmetric.
• physical
  • met
    Someone who you have actually met in person. Symmetric.
• professional
  • co-worker
    Someone a person works with, or works at the same organization as. Symmetric. Usually transitive.
  • colleague
    Someone in the same field of study/activity. Symmetric. Often transitive.
• geographical (pick at most one)
  • co-resident
    Someone you share a street address with. Symmetric and transitive.
  • neighbor
    Someone who lives nearby, perhaps only at an adjacent street address or doorway. Symmetric. Often transitive.
• family (pick at most one)
  • child
    A person’s genetic offspring, or someone that a person has adopted and takes care of. Inverse is parent.
  • parent
    Inverse of child.
  • sibling
    Someone a person shares a parent with. Symmetric. Usually transitive.
  • spouse
    Someone you are married to. Symmetric. Not transitive.
  • kin
    A relative, someone you consider part of your extended family. Symmetric and typically transitive.
• romantic
  • muse
    Someone who brings you inspiration. No inverse.
  • crush
    Someone you have a crush on. No inverse.
  • date
    Someone you are dating. Symmetric. Not transitive.
  • sweetheart
    Someone with whom you are intimate and at least somewhat committed, typically exclusively. Symmetric. Not transitive.
• identity
  • me
    A link to yourself at a different URL. Exclusive of all other XFN values. Required symmetric. There is an implicit “me” relation from a subdirectory to all of its contents.

List from: http://gmpg.org/xfn/11

Do you think this has any use? Do we have to inspect the source code tags just to know the relationship of one site to another? Maybe these tags are ahead of their time that’s why?

So far the only application I have seen that uses this is Google’s Social Graph API. Actually I haven’t even seen a working application of this API but just the knowledge of the API exist. View the video below and learn more on this API. Now in SEO contest like SEOContest2008 and Yicrosoft Directory, if you link to your competitors, what XFN is that? Opponent? Enemy? *LOL*