I was impressed with the research behind this article and their conclusion:

Social Media
Effects of Twitter in an online environment http://www.elearnmag.org/subpage.cfm?section=articles&article=154-1
“First, there was much enthusiasm and participation noted by professors who use Twitter that was not echoed when a particular graduate student population was surveyed, this should be investigated. Speculated reasoning for this includes that if students are not already used to using Twitter it may appear as an extra chore or homework assignment for which they receive no reward. Second, should Twitter, or any social networking site for that matter, be pushed if students are satisfied with discussion boards? Possibly over the next few years, as LMSs develop and incorporate the ability to contribute to social networking sites from within, use will increase. However, for now, it would seem discussion boards are the place keep in-class participation.”