Recently, one of my students evaluated this tool - Diigo. It was listed 22nd out of the top 100 Web 2.0 Tools for 2009 by Jane Hart.

This free tool would make an excellent addition to a WebQuest as you can have an Educator's package which allows teachers to have their class isolated and sharing content with each other. The point of this social networking tool is sharing of information. Students can come into a site and use the highlight or sticky note about the information on this website. Other students researching the same issue/website can then read what another student has thought.

A great way to learn collaboratively!