Bakshy, Marlow, Rosenn, Adamic. The Role of Social Networks in Information Diffusion.
"At the time of the experiment [8/4/2010 - 10/4/2010] there were approximately 500 million Facebook users logging in at least once a month. The experimental population consisted from approximately 283 of these users."
Information diffusion in social networks: related research articles on Google Scholar.
