"When Pear Analytics started its short-term study, it assumed that most of the tweets would be either spam or self-promotion. This belief, it said, was driven by the growing number of firms starting to use Twitter as a tool to drum up sales.
Instead, it found that 40.5% could be classified as pointless babble, 37.5% as conversational and 8.7% as having pass-along value. Self promotion and spam stood at 5.85% and 3.75% respectively."

BBC NEWS: Twitter tweets are 40% ‘babble’