Here are a few ways that I could see Diigo being used in a class environment and personal/professionally...
Course resource - It is easy to create groups and groups lists of bookmarks. A class could aggregate a collection of online resources for a particular project, complete with annotations and notes that the entire class could share.
Personal Learning Network - A person can use this tool as part of their own learning network, gathering resources, sharing them and connecting with others that share the same interests. I use the 'Read Later' list to automatically populate my Google Reader. This allows me to bookmark a site and read it on my own time.
Backup - Diigo can serve as a place for you to backup all the bookmarks you have saved in your browser. Your computer of course will never crash, but in case it does it would be nice to have your bookmarks saved somewhere other than your computer.
If you use Diigo or another social bookmarking tool, please comment sharing how you use the tool.