As wikipedia well defines, a social network service is "a platform that focuses on building social relations among people" and I sometimes have wondered what a social network is and what is not. Let us go through it.
Some well known examples of online social networks are: Facebook, Linkedin, Xing, MySpace, Twitter, Tuenti (mostly in Spain and recently acquired by Telefonica) ...
An online social network intends to replicate the real world relationships and as, in real world, every person may be related to those with similar interests and likes, they also may be related to others who are contacts of the their contacts or with a shared past, like same university, same home town, whatever that would make you have a relationship of any kind, may be a good reason to keep such relationship in an online social network with the difference that the geographical distance does not play any role, one can be related to any person with internet access.
An online social network should provide users with some services:
:- a blog where to share status, links, pictures - wall on Facebook, timeline on Twitter, professional profile on Linkedin, ...
:- a messaging mechanism
:- a way to make and delete a friends - followers on Twitter, contact on Linkedin, ....
:- a way to group some individuals - lists on Twitter, groups on Facebook and Linkedin
:- a mechanism to manage the visibility of the information and messages shared
:- optionally, some platforms provide a chat engine.
Well, according to the above written, the instant messenging could be a social network or even the sms of your mobile ... the only difference I see is the usability of Facebook, Twitter, Linkedin and the other guys is much better than your mobile ;-)