http://mobile-learning.blog-city.com/devising_a_classification_matrix_for_us
As some people might be aware I've got my teeth into Twitter at the moment. As part of this, I'm wondering if I could devise a robust-ish classification index for the different types of Tweets. The intention is to use this as a way of introdu

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As well as AJ's comments on locatltion and hashtag/topic, there were a bunch of other uses that people pitched in for my recent session on microbloging for the DCDI conference. Check out:
http://flickr.com/photos/josiefraser/2438183007/in/set-72
http://flickr.com/photos/josiefraser/2438183303/in/set-72
These included breaking news relay (especially famous people dying for some reason), developing/cultivating relationships, explorin/testing ideas. I'd be careful with your "direct 1-2-1 communications with other people" - while messages are sent for the primary attention of one other person, the context of sending them openly rather than DMing means that very often direct messages are sent to one person but addressed to a (usually unspecified or unspecifiable) larger audience. Good luck! I couldn't reply over at your blog for some reason - nice to talk about it over here though.