Another snippet from Duncan Watts' excellent Six Degrees: The Science of a Connected Age book (now in paperback, Amazon / Barnes and Noble / Powell's):

"[I]n the past, networks have been viewed as objects of pure structure whose properties are fixed in time. Neither of these assumptions could be further from the truth. First, real networks represent populations of individual components that are actually doing something -- generating power, sending data, or even making decisions. Although the structure of the relationships between a network's components is interesting, it is important principally because it affects either their individual behavior or the behavior of the system as a whole. Second, networks are dynamic objects not just because things happen in networked systems, but because the networks themselves are evolving and changing in time, driven by the activities or decisions of those very components. In the connected age, therefore, what happens and how it happens depend on the network. And the network in turn depends on what has happened previously. It is this view of the network -- as an integral part of a contiuously evolving and self-constituting system -- that is truly new about the science of networks." (28-29) (emph. in original)