I'm no expert but think social bookmarking and CiteULike could be used very differently. For the CiteULike repository I worked with students on last year, it was a compilation of about 500 references (not URLs - they were sorting through the literature/evidence for health IT evaluation studies. We worked in EndNote first and then after many iterations, uploaded the EndNote file to CiteULike. That seems fairly different (to me at least) from the kinds of things we are doing here in Diigo. In theory, I guess you could use them both for similar things, but I would save CiteULike for referencing/citations and Diigo for web sites. Just my $.02
It's time to head back to school and there are a number of web-based and social tools to help you get through the school year. Here are 15 essential ones.\n