I just had the greatest time working with a group of fantastic 3rd graders using a program called Edmodo. Think social network and you have the idea but with an educational twist. The teacher creates an account, names their group, and then has their students build accounts to join Edmodo. The neat thing is students do not need email accounts and you control who enters by having a group code to give to those you want to invite into the conversation.
Today's class was starting plant study and research so their objective after creating their login was to post a question they had about plants. Right away the student's post appear and the other students are able to respond with an answer or by posting additional questions.
Edmodo would be a great tool to use to engage students while watching a video or student presentations and allow everyone to become actively involved by back channelling.