Many people use Twitter as a backchannel, but there are some people who would prefer a different solution – because, otherwise, audience members must obtain Twitter accounts participate, which they may not want to do, and the conversation may be more public (i.e., broadcast to the entire universe) than some people might like. Here are five alternatives for a classroom back-channel.
http://www.freetech4teachers.com/2010/01/five-platforms-for-classroom-back.html