What qualities do you consider unique to a "social network" system (beyond its own self-marketing or the time-period in which it arose) that aren't qualities already present in ICQ, E-mail, IRC, newsgroups, etc?
One distinct quality is that as a user you actually have the possibility to navigate the network. On ICQ you only see your own contact list. You cannot see how many friends/contacts/followers/relations anyone else has. You cannot traverse the network from your direct friends to indirect friends.
A social network puts the connections you have with other people prominently on display for everyone to see. The number of "friends" you have is a central social status metric on social networks.
