There are going to be so many permutations and approaches to making sense of private tags and public tags (where “private” = done for oneself, but still accessible by the public). This will all intersect with social networking software, analyzing tag usage among users who know one another and users who are share interests and demographics. We need, imo, room for both public and private. That’s why I’d like social tagging sites to allow me to designate a tag as private.

David Weinberger commenting in his What is a folksonomy anyway?