Five Advantages:
1. Extension/increase of/in “community” of the classroom (especially if it extends beyond class time).
2. Response and class participation.
3. Method to help facilitate group projects.
4. Captures and involves nonverbal students who are(!) interested.

Five Disadvantages:
1. Distracting for some students, whether in classroom or as an outside assignment.
2. Some may not have technology available (depends on the institution, relative wealth of student body).
3. Loss of control of task, if in classroom (side Twittering going on).
4. Usage of grammar/citations/sourcing in 140 character streams.
5. Unmotivated or technically-challenged students.