I was in a meeting today and somebody was asked, "What's the difference between a conference and an exhibition?"
The reply was, "well, for example i see an exhibition as all your Facebook friends, however a conference is better described as a Facebook group, because it's more specific, it's about a particular interest group."
I really like this metaphor and think it sums up the difference well. Social networking is 'social' networking because it puts already existing social concepts into another place that's easily accessible, online.
This transfer of an existing interaction onto the internet perhaps goes part way to explaining the huge popularity of social networking at the moment. Nothing much had to change in people's thinking.
The less somebody has to think the better something works, and i mean that for anything! Websites, instructions, social networking sites, online networking...