Agreed. Besides the length issue of a tweet (which don't apply to blog posts, as we all know from Steve Yegge) I also think that there is no incentive for a researcher to participate in the social web--at least none that I can think of. Actually, quite the contrary is true: yesterday's post on academic blogs linked to the "serious engineering" blog, which contains the following line in a post about time management:

  Do not read blogs and above all do not write them.

AFAIR, Matt Might had a somewhat different point of view, where he elaborates on strategies how to publish blog posts out of presentations, etc. To me the social web compares very well with the TV on the time-sink scale: you save a lot of time by not doing it.