According to the column (which I encourage you to read):
The Fifth Estate includes specialized Web sites, online news providers, bloggers, aggregators, commentators and much more, a virtual buffet for consumers hungry for news and information. But for all its innovation, the Fifth Estate is still heavily dependent on content generated by the Fourth Estate.There's a lot more, but I hate to cut-and-paste when you can just check it out yourself. The point is, this new, emerging, dynamic, ethically-challenged Fifth Estate may very well revolutionize how the Fourth Estate operates. And, obviously, this will affect what people know. And how people come to know it.