22% of tweets include a URL

"- 22% of tweets include a URL (‘http:’)
- 36% of tweets mention a user in the form ‘@user’; 86% of tweets with @user begin with @user and are presumably a directed @reply [or 'mention']
- 5% of tweets contain a hashtag (#) with 41% of these also containing a URL
- 3% of tweets are likely to be retweets in that they contain ‘RT’, ‘retweet’ or ‘via’ (88% include ‘RT’, 11% include ‘via’ and 5% include ‘retweet’)"
Source: Stowe Boyd citing statistics from a Microsoft draft report by Dinah Boyd et al, 20th June 2009
Full report here - but beware - it's draft only, and not final results or conclusions