Hierarchies are just another type of network -- mathematicians call them "trees". Trees are simple networks, without cycles (closed triangles). One can look at an organization as consisting of "prescribed" wiring (hierarchy, matrix org., etc.) and "emergent" wiring (communities of practice/interest, mentoring, advice/KOL/SME networks, etc.).

Here is a diagram of an actual organization, showing both hierarchy and emergent work flow together... http://orgnet.com/Wirearchy.pdf