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
Here are the top 20 largest social bookmarking sites ranked by a combination of Compete and Quantcast data. For each site, we show unique U.S. monthly visitor data as well as respective rank.
B. Krause, R. Jäschke, A. Hotho, и G. Stumme. HT '08: Proceedings of the Nineteenth ACM Conference on Hypertext and Hypermedia, стр. 157-166. New York, NY, USA, ACM, (2008)