My Take
Diigo used to be my favorite go-to social bookmarking site for any and all research that my students conduct. Diigo even publishes bibliographical information for you, too. As of December 2014, however, Diigo was purchased by an overseas company that is quickly monetizing the site, limited free services and actually taking away previously useful features. If you want to have all of the features that you were used to, then you have to pay.Because of this event I now suggest using Annotary, a brilliant alternative that is more graphically attractive and user-friendly.
How I Use It
Diigo lets you highlight, sticky note, and share (including sharing your highlights and sticky notes) any website that you wish to. More than a bookmarking site, Diigo stores webpages for you so that you will always have the websites at your fingertips. I used to use it (see below) for my own research, for individual student research projects, and for small group research in my history classes. Students found sharing websites and notes to be useful. That being said, please read my take, below.