Does posting articles on social bookmaking build links and how does it compare to submitting articles to article directories
I think social bookmarking is more effective link building than submitting your article to article directory and the approval time may take long. Social bookmarking can boost your traffic than your article in article directory.
Social bookmarkings will definitely boost traffic to your site though not for your PR as they use a nofollow attribute...still both will help you...some article directories are really strict they usually have minimum on there articles and it'll be checked first by their editors so it might take long not like in social bookmarkings...
Digg and Delicious the two most popular ones, they dont have no-follow. I heard Netscape's version is doing pretty well for backlinks.
I checked my links in Google Webmaster tools and I have quite a few listed coming from both Digg and from Stumbleupon. I guess that means you can add Stumbleupon to the list of social bookmarking sites that don't have no-follow.
Delicious uses nofollow. For me its all about magnolia. For some reason, some of the articles I send there explode and get bookmarked by hordes of people. Others get completely ignored.
For me stumble rocks. I have a little reputation on the community but each of my posts gets atleast 100-140 visitors from SU. SOme of the posts explode as all of you said.