... any CMS managing any sizeable amount of data needs to support trees. Anything else will lead to an unmanageable mess. However, systems with smaller sets of content, especially with a smaller group of authors, can get away with tagging as well. Facetting only really works well with a system that stores content that is highly structured at least on a per node type basis.

http://www.sitepoint.com/cms-content-organization-structures-trees-vs-facets-vs-tags/