R. Dr. Asher Meir was asked the following question (link):
Blogs provide an open forum for readers' comments. Is the blogger responsible for encouraging slander and other irresponsible contributions?Here is an excerpt from his response:
However, we have to make a suitable exception in the case of public figures or aspects of a person's life which are intentionally opened to the public. When someone runs for public office, he surely expects, even wants, others to openly discuss his qualifications for office, whether positive or negative. Likewise, if someone makes a public speech or publishes something it is fair to assume that he is willing to have his ideas weighed in the "court of public opinion", with its self-appointed lawyers for defense and prosecution alike. Any serious scholar is grateful for the insights gleaned from critics...So, it would be perfectly fine for me to say that I think Dr. Meir is wrong and doesn't know what he's talking about. Except that he isn't and he does.
Blogs are not an appropriate forum for mentioning the virtues and foibles of unassuming people we encounter in everyday life. These people don't seek our praise and are justifiably mortified to be criticized in the public square of cyberspace. However, public figures must, and generally do, reconcile themselves to the fact that their message will be lacking in consistency and impact if they don't open it to public debate. Bloggers may generally assume that these individuals are willing to be discussed on blogs as long as basic standards of journalistic ethics are maintained, including attribution of facts, right to make a reply, and so on.
Also see this post.
UPDATE: Let me clarify that Rabbi Dr. Meir is not discussing whether or not a blogger is allowed to repeat lashon ha-ra about another person, nor is he stating that one is allowed to post lashon ha-ra about public figures on a blog. He is discussing the rabbinic prohibition of avak lashon ha-ra, discussing someone by which one might lead others to speak lashon ha-ra. Must a blogger be concerned that his readers will speak lashon ha-ra about a subject of a blog post. To this, Rabbi Meir answers that the blogger needs to be concerned about private individuals but not public figures -- providing one abides by basic journalistic standards.