While an individual user may use Twitter primarily as a conversational tool or a broadcast medium, in its totality, Twitter operates a lot like a wiki: as a knowledge-sharing, co-creation platform that produces content and allows its consumption. Conversation is perhaps the most simple and obvious form of collaboration, but would anyone claim that Wikipedia is a conversational platform? Despite the presence of information sharing, co-creation of an end product, and even discussion pages, Wikipedians on the whole aren't having conversations. According to this argument, Twitter is no more a conversational platform than Wikipedia is.
Mashable write up of our Twitter article: Alan Cann, Jo Badge, Stuart Johnson, Alex Moseley. Twittering the student experience. ALT-N, Vol. 17, October 2009. http://newsletter.alt.ac.uk/xrctg5ovlfkimsphpsy77s