I do not want to steal Mr. Byrne's thunder here, but I do want to make my teachers aware of his wisdom. So am sharing this to encourage you to look at his blog-post for yourself.
One of the most common questions people ask me is, "What do I do with a blog?" That is actually a tough question for me to answer because I there are so many different options that my mind begins to draw a blank when I am put on the spot. However, I will more than likely respond with Richard Byrne's response from this point forward. His suggestion is to use it as a place to perform a weekly reflection upon what they learned. This would give you an opportunity to build your vision as to what you want your class blog to become.
I completely understand how difficult it is to visualize prior to having a blog. I think it is something you figure out as you go, learn from your mistakes, and also learn from others' mistakes.
On a side note, many people ask me what you do with a Twitter account. I should immediately respond with, "Follow Richard Byrne."
via Free Technology for Teachers: A Simple Yet Powerful Student Blogging Activity.