So far, keeping track of your social relationships on the web has been something that primarily happens on the big social networking sites — Facebook, MySpace and Friendster. They all have different features, but the common thread between those sites is that you can define and manage your relationships with other users on each site.

But what happens to those relationships when you leave Facebook and go surfing around the larger web? How do you stay in touch with your friends and continue to manage those relationships once outside the sealed ecosystem of a social network?

And as popular as they are, we all have a few friends who purposefully avoid the big social networks for whatever reason. Even those people have loose systems for defining their relationships — maybe they have a blog and they keep a “blog roll” or a list of sites they like in the sidebar. It’s much less formal, but it’s analogous to the friends list in a site like Facebook or Friendster (just don’t point that out to them, lest they become even more reclusive).

The web is a big and complicated system of connected data, so surely there’s a structured way to define and manage relationships between people without joining a social network. And in fact there is.

It’s known as XFN, or the XHTML Friends Network. Using a series of different rel tags in your markup, you can indicate who your friends are what the nature of your relationship is.

While not officially part of Microformats, XFN uses some microformat components to define relationships, and the two groups share similar goals.