From my perspective and experience, social networking is all about connections and relationships using a variety of mediums.
In Himanen's book "The hacker ethic and the spirit of the information age", he talks about the work culture of the programmer who lives and breathes their passion. To me, social networking is very similar to the connections and relationships within the programming community. When a programmer is "in the flow" they can't stop thinking about their project.
People that are active in various social networking environments are the same. They are always thinking "what can I share", "who can I connect with". With the development of smart phones, the ease of use, intractability and ease of connection has made the environment addictive for many people. For the school librarian, the use of these tools enables us to reach out and connect with our clientele. The electronic environment, allows us to gather metrics and respond in a timely manner.
My first experience with social networking was in the early 90's with my exposure to bulletin board systems (BBS) when I was doing computer science at Melbourne Uni.
Whilst there is no hard evidence on how this phrase embedded itself into our vernacular, there is no doubt that it's infiltration is aided by what Himanen has coined as the "hacker work ethic".