19% of tweets, and 57% of ReTweets contain a link

"Over the past few weeks I’ve begun delving into much deeper analysis than I have in the past with more advanced tools and a much larger dataset. At present I have a database of over 10 million ReTweets and I’ve gained access to Twitter’s new streaming API which allows me to build a very large (10 million and growing) random sample of all tweets (not just ReTweets).
In re-visiting a data point that I looked at 6 months ago (this time with a larger data set), I found that in a random sample of normal (non-ReTweet) Tweets, 18.96% contained a link, whereas 3 times that many ReTweets (56.69%) included a link"
Source: Dan Zarrella in his blog, 1st July 2009
Earlier - 22% of tweets include a URL (using a different data set, and probably including retweets)