Yong-Yeol Ahn, Sebastian E. Ahnert, James P. Bagrow & Albert-László Barabási
http://www.nature.com/srep/2011/111215/srep00196/full/srep00196.html
This paper was highlighted by Scientific Reports as the most downloaded paper on all of nature.com for the month of December, 2011 (including papers published in Nature and the Nature-branded journals).