[Ed. I wish someone would do a study on what percentage of tweets are news about twitter.]
BBC NEWS | Technology | Twitter tweets are 40% ‘babble’
http://news.bbc.co.uk/1/hi/technology/8204842.stm
Hot from the retweetery, the BBC is reports that Pear Analyitcs has done a study showing that 40% of all twittering is babble. Another 37.5% is conversation, the rest is mostly self promotion and spam.

