Twiducate is a free platform for creating your own micro social network in a Twitter-like format. Twiducate allows you to create a private network for posting assignments and messages to your students or other people you invite into your network. As the creator of your network you create and administer the accounts of the students in your network. Students in your network can reply to your messages and to the messages of other students.
http://www.freetech4teachers.com/2010/10/twiducate-social-networking-for-schools.html