Source: Shaomei Wu, Cornell University, Jake M. Hofman, Yahoo! Research, Winter A. Mason, Yahoo! Research, Duncan J. Watts, Yahoo! Research, 'Twitter Flow' 28th March 2011
(Includes a link to download the full research report)
Note 1 - Why just just looked at tweets with links (& in fact just tweets with B.t.ly Urls:
From the full pdf:
"In addition to the follower graph, we are interested in the content being shared on Twitter|particularly URLs|and so we examined the corpus of all 5B tweets generated over a 223 day period from July 28, 2009 to March 8, 2010 using data from the Twitter \ rehose," the complete stream of all tweets2. Because our objective is to understand the flow of information, it is useful for us to restrict attention to tweets containing URLs, for two reasons. First, URLs add easily identi able tags to individual tweets, allowing us to observe when a particular piece of content is either retweeted or subsequently reintroduced by another user. And second, because URLs point to online content outside of Twitter, they provide a much richer source of variation than is possible in the typical 140 character tweet. Finally, we note that almost all URLs broadcast on Twitter have been shortened using one of a number of URL shorteners, of which the most popular is http://bit.ly/. From the total of 5B tweets recorded during our observation period, therefore, we focus our attention on the subset of 260M containing bit.ly URLs."
So in fact the title should say "50% of the potential impressions generated by tweets with links are generated by just 20,000 users"