Watch a typical whole group discussion in the classroom and you'll most likely see a "hub / spokes" flow of information. Teacher to student A and back to teacher. Teacher to student B and back to teacher. So it goes as the "bluebirds" get to show how smart they are.
Over time, students learn that their comments are of provisional value until "approved" by the teacher. That's because in this style of discussion the teacher is most likely searching for specific replies - sort of playing "guess what I'm thinking" with the "best" students in the class.
Students tend not to listen to each other and only focus on what the teacher says or validates - "will that be up on a test?" When students are put in small group discussion, they rapidly get off subject. With no teacher to validate their comments, they naturally gravitate to other subjects where peer comments are valued - "what are you doing this weekend?" Often teachers then conclude that small group discussion doesn't work.
In my workshops I train teachers in discussion techniques that foster student reflection and interaction. The strategies are focused on getting the teacher out of the role of information gatekeeper and encouraging student-centered dialogue.
With practice, teachers find that students are eager to engage and participate. We know they want to contribute, because outside the classroom, students are flocking to social networks to share their thinking with one another. It's unfortunate that our students can't be part of the (offline) social network sitting beside them in class.
"UT Dallas History Professor Dr. Monica Rankin, wanted to know how she could reach and include more students in the class discussion. She had heard of Twitter... The following is a short video describing her "Twitter Experiment" in the classroom with comments from students about the pros and cons of Twitter in a traditional learning environment." (Filmed by UT grad student kesmit3.) Link to notes on the experiment.