http://en.wikipedia.org/wiki/Friend_of_a_friend

It's one of the most crazy things I've seen about this web. FOAF is a standard to define your relationship with another person, as friend.

I have been writing some sort of Web 2.0 site and I am still thinking if I should use FOAF. However, seems it is just for sake of telling people I know this protocol than having real usage on it, at least today.

Anyway, it's interesting to know about this XML protocol for friends.