Social Network - Definition

In " Social Network Sites: Definition, History, and Scholarship" Danah Boyd and Nichole Ellison Define a Social Network Sites as:

Web-based services that allow individuals to:

1. construct a public or semi-public profile within a bounded system,
2. articulate a list of other users with whom they share a connection, and
3. view and traverse their list of connections and those made by others within the system. The nature and nomenclature of these connections may vary from site to site.

I initially couldn't understand the motivation for this definition, but now I get it (I believe). Looking back at a computer network and thinking of "nodes", this all makes sense.

1. Establish a visitable node (as yourself)
2. Dictate connected visitable nodes (your friends)
3. Traverse to and through the subsequently connected nodes. (friends of friends)

As definitions go, you've got to draw the line in the sand somewhere... I got it.