My kids think that Twitter is the preserve of middle-aged people. The youth all use Facebook. From discussions with my students, this kind of impression has been confirmed for their generation too. I talked a bit to my Historical Jesus class last semester about Twitter and was surprised to find out that very few students in the class even had a twitter account. Our student paper here at Duke, The Chronicle, had a nice piece called Tweet tweet. What's the Rage? this last semester, but discussions with students suggest that Twitter is hardly on their radar while Facebook is their natural hunting ground. When I suggested that one of my advisees talk to another student who shared the same research interests, I said that I would look out her email address and she replied, "Don't worry; I'll just find her on Facebook". The average undergraduate apparently has over 900 friends on Facebook.
This is all a long-winded way of saying that I don't think that undergraduates are embracing Twitter yet to the extent that it can provide a useful venue -- for me -- for supplementing teaching. The time may well come, but I like to work with things that students already have some familiarity with, like Instant Messaging, which I still think provides a great way of communicating outside the classroom.