There was a lot of discussion not too long ago when Wikipedia decided to Nofollow its links. It really is a bit sad, because if the theory is that the community makes the content accurate, then the links should not have to use this attribute.
I suppose that the counter-argument is that fighting people looking for SEO value in links was making it harder to keep the content accurate. Hard to argue with that, although it does point to an underlying weakness in Wikipedia.
However, many people may not realize that Nofollow is used on other social media sites.
For example, del.icio.us uses the Noindex and Nofollow tags on all of the pages of its site. StumbleUpon also uses the Nofollow attribute on all the links on personal profile pages of its site. StumbleUpon does not use the Nofollow attribute on links that appear on its Popular pages. However, links on those pages do not remain in place very long.
The use of Nofollow on social tagging sites does make sense. After all, putting a link in your own profile is easy to do, and every webmaster is going to link to their own sites. Also, these sites want to foster an environment not altered by the pursuit of SEO benefits.