Indeed very interesting study.
However, I trust that the system will 'evolve'. yes, one comment from non-twitterer was that 'he was not 'vain''
at present it might be a broadcast system, but there might be other cases that promote/enable more 1-2-1 communication, like how I use it, my feed is private and only people I know/interested in or met can see/interact with me.
sadly my usage data is not available to you.
therefore, I trust the system will evolve and when it address 'privacy' and private group aspects, it will be much more ubiquitous and powerful tool!