Consider:

One of the most popular tags created on the bookmarking service is "to_read," a tag attached to links of pages users want to remember to read.

"There is a behavior around tags that has nothing to do with categorization," Joshua Schachter [creator of del.icio.us] said.

Moreover:

A user who travels to Tokyo might tag all photos from the trip as "Tokyo," including those taken inside a hotel room, Stewart Butterfield [creator of Ludicorp] said. But other users might expect to see only photos of the Tokyo cityscape, and not a hotel's interior, when viewing photos tagged as Tokyo.

Tags and folksonomies are just an over-hyped collective delusion. Just like the semantic web.