Newspapers have an editorial pages. In these, anybody can submit their thoughts or feelings about modern day issues or concerns. The problem with these pages is they are limited in space, and the editor ultimately decides what is published and what is not. Blogs are like editorial pages, but anyone can publish anything on their own site with unlimited amount of space available.
Martin Luther King's letter from Birmingham jail is a very well written essay describing the difficult position of black American's before and during the civil rights period of American history. Had this letter been written today I am sure it would have been published as a blog post, in fact, I am sure many people would have responded to his blog post with posts of their own.
The letter takes on a blog's tone. He has studied and more important, he has experienced many of the atrocities he describes in his letter. He gives his opinion of what he observes, makes an emotional connection with the readers, and gives a rallying call to encourage others to join in the fight. Many blogs today take on similar issues with the hopes of gaining support for their cause.
Martin Luther King has been dead for more than 40 years, but I am sure, were he alive today, he would be using a blog to define, educate, and recruit people into his cause.