If you are interested in micoblogging services like Twitter you may be interested in Edmodo. Edmodo described themselves as:
a private microblogging platform that teachers and students can use to send notes, links, files, alerts, assignments, and events to each other. Teachers also have the ability to mark any post from their classes as public to share with the outside world. You can also create private groups for you and your colleagues to collaborate. It’s free & simple to use!