The latest advancement in Twitterized politics: Organizing for America has created a page that will help visitors tweet at their senators (OFA recommends you ask for a "yes" vote on health care reform). Enter a zip code, click "tweet," and the site opens up a Twitter page with a pre-fab health care tweet all set to go, including hash tags that let OFA trace all the tweets published through its service.
For instance: "To @SenatorCollins: Please pass health insurance reform this year http://bit.ly/3133JC #hc09 #ME #04101"
Since such tweets are visible when users search for a senator's Twitter account, it's more traceable and public than calls to a member's office. Formally, it's like clicking to send a pre-written op-ed to a local paper. Senators not on Twitter are immune.