I haven't included the use of diigo in my course, but I'm considering having the students include web sites in their discussions. I'm rethinking now and considering the use of diigo in the class. This web site had some instruction on best practices for using Diigo and I could point the students to this source.
Online Learning:
Remind students of intended objectives
Encourage/teach how to save/bookmark information
work online into the normal fabric of student learning