• Research  ~   annotate, archive, organize
• Share ~  build a personal learning network
• Collaborate  ~ create a group knowledge repository 

  Note:   here are some old video intros.  Although certain interfaces have changed now,  they might still be quite helpful:

• "Social Bookmarking:  Making the Web Work for you"  ~ by Emily Barney
• "Diigo V3: Highlight & Share the Web! Social Bookmarking 2.0"
• "Diigo V2 ~ Improving how we find, share and save information"