Flickr and Wikipedia among others are sites that append the rel="nofollow" automatically to all external links. What does this mean and how does it relate to the free web?
First of all the rel="nofollow" attribute is added to hyperlinks to prevent search engines using the link between sites as part of their ranking process. In some search engine routines a link from page A to Page B is considered a "vote" for Page B by Page A. Google uses this method, although it has further routines for calculating the voting power of a link compared to others (a link or "vote" from a more important site has higher authority or "voting power" than one from a minor site).
In strict terms I guess the search engine should not follow the link to Page B, although some do and some don't. What is clear is that the big players in search technology (Google, Yahoo, Bing) do respect the notion that links with the rel="nofollow" attribute have no "voting power".
Why was such an idea considered necessary? In order to overcome the havoc to search engine results that could occur by people posting comments that include links to a large number of sites, e.g. blogs, photo-sharing websites, etc. In fact the attribute was the brainchild of a Google/Blogger sharing of minds before Blogger became part of the Google empire.
In an ideal world there would be no need for such a tag. Spamming would be removed by caring blog owners (or blog platform operators), or could be corrected for by the search engines (this comment is spam, I will ignore the link). Unfortunately neither of these is entirely possible, or entirely foolproof.
Blanket use of rel="nofollow" however seems a bit mean. For example I use Flickr regularly and I also blog regularly. Strangely enough sometimes I even post photos and blog about the same thing! When this happens I tend to link from my blog to my Flickr photographs, and from some of my Flickr photographs back to my blog.
When I link from my blog to Flickr my votes count. My blog actively increases the importance of my photographs (at least as far as search engines are involved). When I link from my photos to my blog however Flickr automatically adds a rel="nofollow" attribute to my links. The importance of my photograph cannot be shared with my blog. Flickr keep all of its voting power for internal site links (a great way to increase its importance at my expense). I should point out that this is for photograph, set and collection descriptions - not only comments.
In this way Flickr has become a PageRank super-sink - it pulls in importance from thousands and thousands of sites across the web, and gives nothing in return. It's the start of a uncontrollable PageRank monster. In fact I have started calling Flickr 'The Great Importance Attractor' - probably because I like physics and maths too much.
Is this fair? No, of course it isn't. I'm not too fussed if the links in comments on my photographs and blog posts get their votes removed. This is where the majority of spam comes from (besides forums I imagine). But my content? I think I should be able to link it together in a way that doesn't contribute to the making of PageRank monsters.