Has anyone explored the design space that Twitter is a member of? It seems like it's just a cut-down version of publish-subscribe with a couple of eponymous channels (write, read) per user (and then the complexities of message threading).

The invention of hashtags was an attempt to add adhoc pub-sub channel support (not, as they are seemingly sometimes assumed to be, a metadata/folksonomy thing or an opportunity for a pithy closing phrase.)

All this is easy to duplicate. Is any of it even patented by Twitter?