Twitter, technically cannot be called a social network despite the several million @replies and followers that exist. That is because it is not necessary to mutually befriend users, unlike Facebook or MySpace.
A recent study looked into this aspect of Twitter and this was the observation:
Only 22.1% of user pairs follow each other on Twitter.
How good is that? Here is how it compares to
Flickr : 68%
Yahoo 360 : 84%
Cyworld Guestbook : 77%
So, I would tend to call it as a platform for social dissemination.