This folksonomies article is good for the questions it raises, but alsofor the data it collects in one place -- lots of good statistics ondel.icio.us and flickr usage of tags in this paper:
Folksonomies: Tidying Up Tags?
"Thisarticle looks at what makes folksonomies work. The authors agree withthe premise that tags are no replacement for formal systems, but theysee this as being the core quality that makes folksonomy tagging souseful. The authors begin by looking at the issue of "sloppy tags", aproblem to which critics of folksonomies are keen to allude, and ask ifthere are ways the folksonomy community could offset such problems andcreate systems that are conducive to searching, sorting andclassifying. They then go on to question this "tidying up" approach andits underlying assumptions, highlighting issues surrounding removal oflow-quality, redundant or nonsense metadata, and the potential risks oftidying too neatly and thereby losing the very openness that has madefolksonomies so popular." Commentary by Marieke Guy and Emma Tonkin,UKOLN. [D-Lib Magazine]
(You gotta hand it to the old-school Digital Library people.)