I have already had much experience using Google Docs. We use it as a staff here at Cornerstone and it is beneficial to help students know what work they are missing and what they still need to complete and turn in. I do know several kids who have used google docs for presentations. It is so wonderful because we continually have problems where one member has all the work and they turn up absent so they cannot work on the project. They also know that it is always saved which is nice.
I have a Diago account but unfortunately have not had the time to get on and learn how to use it.
I have an Edmodo account but rarely get to use it with my students. It is a goal of mine to set something up for next year so students can get on and know what they missed, etc.