From Read/Write Web analysis, we see that the 2 most popular sites are del.icio.us and StumbleUpon. Actually I use 3 social bookmarking site: del.icio.us, furl, and blogmarks. del.icio.us is the richest service in terms of features: you may use networking, publish links to your blog automatically. But the other 2 services have features not available on del.icio.us: Archiving for furl and screencaps for blogmarks.