41% of tweets can be classified as 'pointless babble'

Breakdown of contents, based on a random sample of 2,000 tweets:
"News - 3.60%
Spam - 3.75%
Self-promotion - 5.85%
Pointless babble - 40.55%
Conversational - 37.55%
Pass-along value - 8.70%"

Classification was as follows:
"News
Any sort of main stream news that you might find on your national news stations such as CNN, Fox or others. This did not include tech news or social media news that you might find on TechCrunch or Mashable.
Spam
These are the tweets such as “See how I got 3,000 followers in one day” type of tweets.
Self-Promotion
These are typical corporate tweets about products, services, or “Twitter only” promos.
Pointless Babble
These are the “I am eating a sandwich now” tweets.
Conversational
These are tweets that go back and forth between folks, almost in an instant message fashion, as well as tweets that try to engage followers in conversation, such as questions or polls.
Pass-Along Value
These are any tweets with an “RT” in it.
Now, if there were any tweets that could fit into more than one category (which was rare), if it started with “@”, we deemed it as conversational, even if it was a news item or self-promotion."
Source: Pages 4&5 of Pear Analytics twitter Study, August 2009