Twactivism or Twictivism, call it what you wish, but from what I have seen it seems to be the least useful form of activism short of doing nothing but generates more self congratulatory and communal back slapping than the annual narcissist awards.
Changing your avatar / status message / location to a support slogan or simply retweeting a post is not actually going to make a difference unless you somehow manage to get the retweet seen by someone who is willing to get out of their seat and do something.
However the chances are that they are:
a. Already aware of the problem/event/disaster, so don't really need twitter to tell them
b. Not even checking twitter anyway because they are out of the house making a difference already
Well, unless of course your preferred cause is to save the fail whale, in which case please keep moving along, nothing to tweet here.