Have used Furl for a number of years as my social bookmarking site. I liked the way you could create a MLA bibliography from your bookmarks and keep a copy of the bookmarked website on your account. I stopped using it a while ago due to the number of inappropriate links I was finding in the Most Popular Links page.
I exported all of my links to an HTML file deleted many of the links, then started using Zotero more and more. I have not found anything that comes close to Zotero to help with information management . It keeps a local copy of the website and allows you to markup and annotate that copy. It is, however, machine resident and does not allow for social bookmarking. When you have a lab with individual accounts this works fine but if the students are using one account then the Zotero bookmarked information could become a mixture of multiple students work dealing with unrelated topics (this could be circumvented with proper collection management and tagging).
I had also been looking at Diigo. I had been signed up with Diigo for a while but started looking at in more closely when I saw how you could create slideshow from a list of bookmarked sites (this Diigo slideshow for my students is an example) The one thing I noticed about Diigo is that it might be a one stop center for creating bookmarks. One click saves the bookmark to diigo, furl and Del.icio.ous.
As I started looking at this even more I started wondering if diigo might be used to help with grading student blogsites as you can comment on different parts of an online document. This Voicethread explains what I mean. By the way is it Diego or Deego or what?