Additional insight on the Pew report from one of the most respected thought-leaders on tagging:
I am really happy with the report as it looks at the numbers from a use perspective. Up to this point I have been using tagging service provider numbers (few are made public) along with Alexa hit reports across many services and took that total and divided by the Neilsen report number of total people on the web (approximately 750 million people). This approach provided about .85% of all the people on the web are tagging (does not include tagging on blogs as that is more ad hoc categories, but that is a long post to explain or done over a beer or two).
The difference between the percentages in the Pew report and the numbers I backed into is the Pew is just an American view and mine was looking at things globally. Pew looks at tags and categories and many systems have categories. I am really comfortable with the daily number of 7% on the web are tagging/categorizing and I will likely use that number in future presentations. The 28% number is really surprising, but for one time use it is accurate. This represents a much larger user base than I thought, but is also includes categories with tagging.
Separating Tagging and Categories
The Pew Report on Tagging combines categories and tagging. While optimally it would be great to separate the two out, explaining the difference between the two to a regular person (non-geek) in America will be difficult. Asking if somebody has used certain functionality on a service or one of the 130 or so social bookmarking tools or the many hundreds of products that include tagging will negatively impact the results. The terms tagging and categories combined for a research question make for a question that is more easily answered yes or no.
The Pew Report provides a starting place for future research, hopefully delving into the subject with a little more clarity, where tagging and categories are separated