Social networks were the forerunner of what is now turning into a new term and a new concept, Socialising Networks. It can be seen to be a natural further step in the evolution of Web 2.0.
As the word socialise is defined in wiktionary  the main difference is that the socialising network platforms promote communications amongst members of the network. This means they do not have the primary focus of growing a persons network but of deepening it. There is nothing groundbreakingly new about the technologies involved or the single ideas behind them but it's the combination of networking together with exising communication: email, calendar, IM, forums, blogs, .... This combination adds a whole new aspect to both communication and networks.
This was addressed by Tim O'Reilly in his radar blog .
The communication is also not limited to internet based communications but also mobile phone (SMS, MMS, ...) and real life meetings.
Companies Involved In This Field.
The Ego Effect
Although existing sites social network platforms do support simple internal messaging systems they do not promote communication in the same manner with the principle focus being on supporting the users ego in seeing who can get the biggest network.