Knowledge doesn’t neatly align to a nested hierarchy. Especially written knowledge.

Language is an imperfect means to convey knowledge, and people store that knowledge in subjective and highly personal ways.

You may mentally recall balloons within “entertainment” or “party”, whereas I might store that knowledge under “horror”.

Add onto that the massive focus on using graph theory to scale social networking technologically, and you effectively lose any motivation for rigid hierarchy.

A folder system doesn't have to be strictly rigid, you can still have "symlinks" so the same article appearing in different folders (aka labels if you can easily duplicate content inside folders, but you retain the nested, drill-down approach)

Yes, and it sad the search in this UI doesn’t work…