The issue of visibility is, as Dave notes, important for thinking about new media in general and especially thinking about it historically. Bloggers have helped make visible certain moments or kinds of information (two prominent examples: Presidential sexual follies; racist remarks made by public figures) that may not have become part of the public discussion in an earlier era. In doing so, they have also made visible the ways in which mainstream media had always decided on what was or was not newsworthy, allowing for a more public critique of news institutions as well as politicians.
At the same time (as Fernando points out) we need to realize that media of any kind both open up and foreclose certain opportunities, encourage certain ways of acting and discourage others, bring some kinds of information to the fore and hide other kinds. One of the things that the Internet hides is the physical specificity of the bodies that use it: their visibility. Sherry Turkle has famously celebrated that aspect of Internet communication. By removing physical presence from an interaction, people were allowed to be whoever they wanted to be. If you a middle-aged male accountant from