There is a lovely book entitled Connected : The Surprising Power of Our Social Networks and How They Shape Our Lives by Nicholas A. Christakis and James H. Fowler , PhD.

They distinguish between “organized” networks that you can find (see illustration on right) in bucket brigade, a telephone tree (could also be a hierarchy) and a set of military squads – and open / social networks (below). The number of connects differs depending on the arrangement. For example, in a set of 10 squads of 10 members each the connections would equal 45 per squad (= 450) plus 45 between the squad leaders for a total of 495. All members of the squads are equally close / distant (leaders-to-leaders and member -to-member).
In a open, social network, the number of connections between members is not set – and who you connect with is not set. If you map that network you get something that looks like this (below):

In a telephone tree network the content of the “tree” is based on the message that is given from the centre. Social networks have some unique properties including the fact that they ”
tend to magnify whatever they are seeded with.” Christakis and Fowler go on to say:
Partly for this reason, social networks are creative. And what these networks create does not belong to any one individual—it is shared by all those in the network. In this way, a social network is like a commonly owned forest: we all stand to benefit from it, but we also must work together to ensure it remains healthy and productive. This means that social networks require tending, by individuals, by groups, and by institutions. While social networks are fundamentally and distinctively human, and ubiquitous, they should not be taken for granted.”
To read more see: http://connectedthebook.com/pages/toc/chapter1.pdf