Like you, I never saw Linked In, Plaxo et al as “Social Networks” – they are certainly networks of humans and thus (very probably) obey the same Small World / Scale Free laws of such networks but their purpose – and thus dynamics – is different.

Bebo, MySpace et al however are Social Nets, and (in my recollection anyway) they *started off* as groups of friends who knew each other – but then the “virtual friending process” starts and after a year or so the average buddy list is full of people who you’ve never met – and maybe wouldn’t accept as friends if you did?

In other words my hypothesis would be that F/B now will be a different experience in 1 year’s time once this process operates there as well.

One note I’ d make (being an Old Net Head) is that the “Web 1.0″ SocNets like Yahoo Groups for eg (in my experience anyway) fostered quite a lot of “real life” socialisation once enough interaction had occurred. However, I do not see anything like the transaction traffic on F/B that I see on a typical Group

One hypothesis I also have is that the reason F/B has had such media exposure (in the UK anyway) is because it has been colonised by the (30-something) “chattering classes” to a level that the previous social nets have not – ie its all old news to the kids, but then they don’t write for national newspapers or TV stations.

Also, the other “Social Nets” that tend to be forgotten here they are the games – World of Warcraft, RuneScape et al are played by friendships groups acting as teams, and (my kids) entire class play in particular worlds – more so they can IM each other than anything else.