Readwriteweb.com does this kind of article a lot, and the articles are useful:
"The Social Bookmarking Faceoff"
The social bookmarking market is in a steady state with two dominant players - del.icio.us and StumbleUpon. The rest of the pack, including Yahoo MyWeb, appears to be substantially behind. Will they catch up? In this post we attempt to answer that question.
We also take a look at how social bookmarking has evolved since del.icio.us. (even del.icio.us itself has evolved a lot!). We compare the features and approaches of the different companies, to see which has gained popularity and what has become the norm in this space.