Relationship between number of PubMed citations and number of CiteULike posts per citation. The X coordinates of each point on the plot correspond to the number of different people to post a particular citation. The Y coordinates are counts of the number citations that occur in each of the bins provided by the number of posts. The point at the upper left of the chart shows that more than 10,000 citations (of a sample of 19,118 unique citations) were only posted by one user. This number decreases exponentially as the number of users to post each citation increases.