I'm recommending this thoughtful piece I read called Avoiding the 5 Most Common Mistakes in Using Blogs with Students, from Campus Technology. I liked the way the author differentiated between blogging, online discussion and journaling.
The essential difference between a blog and other online tools is that it is intended to be an individual publication: a one-way monologue or self-post to which others may comment but do not contribute. The original post remains as the person who posted it wanted it to be. This is important to realize in the instructional setting. If a discussion is desired, then blogging would not be the tool of choice. In the same way, if journaling is the intended goal, then an online discussion forum would not be the tool of choice.
[Image: Flickr: "Kid peeking up from the back of a bus"; Uploaded on November 28, 2007 by jim snapper: http://www.flickr.com/photos/jimsnapper/2071609565/]