Review of the various social bookmarking sites

I reviewed Delicious, Digg, Stumbleupon, Diigo, and CiteULike.  My favorite was Diigo because of its team-based features such as group sticky notes and forums, and a group tag dictionary.  I also was impressed with their awesome education groups you could join. 
Possible uses of social bookmarking in libraries is obvious:  bibliographies, quick and easy subject heading links, reader's advisory/information, etc.  For education, the major use that I see is creating protable reading lists and study-guide bibliographies.  It like the fact that a social bookmarking tool emphasizes browsing over searching, so it would be easier for patrons (students and teachers) to use.