the folks over at eBizMBA have put together a list of the top 20 social bookmarking sites. They based their results on data on a combination of Compete.com and Quantcast data
M. Noll, and C. Meinel. Proceedings of the 6th International Semantic Web Conference and 2nd Asian Semantic Web Conference (ISWC/ASWC2007), Busan, South Korea, volume 4825 of LNCS, page 365--378. Berlin, Heidelberg, Springer Verlag, (November 2007)
R. Jäschke, A. Hotho, C. Schmitz, and G. Stumme. Proceedings of the 15th International Conference on Conceptual Structures (ICCS 2007), volume 4604 of Lecture Notes in Artificial Intelligence, page 283--295. Berlin, Heidelberg, Springer-Verlag, (July 2007)
R. Jäschke, A. Hotho, C. Schmitz, and G. Stumme. Proceedings of the 15th International Conference on Conceptual Structures, Sheffield, England, (July 2007)