Blogging is amateur journalism. It is not professional journalism. Bloggers are not held to the same standards as a broadcast journalist because they are not quite as invested in the journalistic process. No money is changing hands because the vast majority of bloggers are not paid. In fact it costs them money to produce their work. This means that most bloggers can't afford to do things we require of mainstream press like fact checking.
Even though there are credible bloggers with great credentials, most are simply guys (and gals) with an opinion and a keyboard. They are no more authoritative, nor likely to be accurate in their assertions, than any randomly picked neighbor. But that does not mean they should be dismissed.
As amateur journalists, bloggers serve an incredibly valuable purpose. Bloggers are sheep herding dogs who feel that their charge is to preserve and disseminate what they see as the truth. The truth, however they happen to see it, is their master and they guard it fiercely.
It works like this.... If bloggers sense that mainstream media journalists are not telling the whole story, they start barking. If other bloggers agree with the originally barking bloggers then they start barking as well. Then they're readers start barking and pretty soon the entire news neighborhood is awake and trying to find out first what the problem is and then how to make it go away. The effect is swift, and sure. Bloggers may or may not be right, but they are the perfect guard dog. They know there is a problem and want to make sure that everyone is paying attention to it. That is amazingly useful. In fact, I think bloggers, serving as an open feedback loop between the government, people, and the press might be the perfect tool for preserving an open democracy.