Twitter

Twitter is extremely different from both an in class discussion and a blackboard discussion. Twitter discussions (as long as they are tagged correctly) can be seen by all people who follow you, who follow the people you mention, and people who click on the hashtags (if any) that are specified. Others who are not involved in the class or conversation can jump on and reply to those who are discussing the topic. It leaves for a very broad network of communication, un-limiting it to the class alone.
Also, twitter (and all other social media sites) will "follow" you forever - pun intended. 5 semesters from now a new student in this class could hashtag something that we used in our discussion. They can then easily find it for reference. Voila! The class comes full circle, even years later!
The benefit of an in class discussion is that you can actually hear and see the way a person is expressing their thought/idea. When reading the status, a lot could be lost in sincerity, passion, or any real emotion. As wonderful as twitter and social media is for social networking, you can lose a lot in translation as well.