I already have an account and frequently use Google Docs. Mostly my documents are shared with other teachers. We have a few documents that are used school wide (staff only) to help make sure students are getting their work completed. I haven't assigned my students to use it in my classroom, but have had some students that used it to work on group projects. I like it because you don't have to worry about the kid that has all the notes being absent on a day! It is also nice because I can monitor and see where their work is going and make notes or comments if needed. The best part about the kids using Google Docs is that you don't have to worry about what program they did their project in, will it work on this computer, did they save it to a place I can get it, etc.
I do have a Diigo account but I have not really had the time to figure it out. I know when I first heard about it it sounded great and I know I want to somehow use it, just haven't been able to do that just yet.
I have also set up a Edmodo account and use it (though rarely) with my Student Council kids. We put up the forms they need throughout the year so they don't have to keep asking us, as well as send reminders about meetings, etc. I would like to practice more with it so that I can use it in my regular class.