Anyway the four most common sorts of pictures are:
- pictures of young women not wearing any clothes
- pictures of young children playing on the beach/in the yard
- participants in the world naked bike ride
- participants in a Japanese religious festival
Clearly naked meant not wearing clothing to everyone who posted these pictures and tagged them that way. That's what we would expect where he meaning of the word is well known in English.
However the tacit metadata (or the associations) of the tag for the various groups were different, ie the way people thought about naked was different. To some it was associated with childhood and innocence. To others it had a sexual dimension, others solidarity and shared action.
And that's the point. In a controlled vocabulary we might distinguish between naked, nude and unclothed depending on context. In a folksonomy that's not the case unless we have a degree of common understanding where to use which synonym to more closely convey meaning.
A folksonomy implies a degree of commonality. For a small group of people working on a shared purpose that's probably fair. For a large random group of people that's not the case, even in such an apparently simple case as pictures of people who aren't wearing clothing.
Where there is no commonality here is no tacit controlled vocabulary and hence you get different classes of images tagged the same, meaning that the tags lose their value as a discriminant.