"TagOntology is about identifying and formalizing a conceptualization of the activity of tagging, and building technology that commits to the ontology at the semantic level."
"(...) analyzes folksonomy metadata for hierarchal semantic relationships via a content analysis of approximately 2000 folksonomy tags in over 600 individual entries. (...) The results indicate that hierarchical relationships are part of Folksonomies."
N. Kuicheu, W. Ning, F. Narcisse, X. De, and F. Siewe. Information Science and Engineering (ICISE), 2009 1st International Conference on, page 2288 -2291. (December 2009)