Backchannels are a useful way to draw more students into a discussion.They can be used during a video, presentation, or skype call to involve the audience in the discussion. Proper ettiquette should be talked about ahead of time. Backchannels should always be moderated. I always tell my students that the whole conversation is being saved. You should expect students to be silly the first time that they use it. Let them play for awhile. The moderator can ask questions that all members must answer.

Some tools are:
Today's Meet

Here is a post with tons of ideas on how to use Twitter in the classroom. Many of these ideas would work with the other tools as well.