I’ll now explain a type of communication flow that you might notice from studying blogs (though not directly from how Flickr interacts with blogs). Among comments listed for a given blog post are often entries that come from other web sites. How is a blog able to track links that come from the outside? Weblogs use linkbacks, a family of methods for receiving notifications of inbound links to a web site.

As documented at http://en.wikipedia.org/wiki/Linkback, there are three major protocols for linkbacks: refback, trackback, and pingback. It’s useful to know which of the protocols are supported by various blogging software so that you know which of the protocols to support if you set out to use linkbacks. Why might linkbacks be useful for mashups? You may want your mashup to either notify web sites that it links to or receive notifications of being linked to.

Note that Flickr doesn’t support linkbacks, although it notifies you when someone else adds a comment to your picture or makes it a favorite.