Microsoft Research social computing team has some thoughts on blogs vs email:

The underestimated strength of blogs is the chronological ordering. The single voice, the public visibility that leads to more care in most blog construction, the feedback are all significant too, but they are remarked upon. Human beings have a tremendous ability to reason instantly and unconsciously about information organized chronologically. If something is 3 months old, we know what kind of information is probably still relevant, what is less likely to be - the shelf life of information is very variable.