Teach them how to blog first. We did an excellent paper blogging lesson first (found on the blog of McTeach), which brought up why we were blogging and how to do it appropriately. This got the students excited, interested as well as got them thinking about what great comments look and sound like.
This is such a crucial step. My own failed attempts at creating a classroom of bloggers can be traced back to this missing step. The paper idea is worth exploring.
I'm really interested in this--want to see if it works with Google Maps. I love to screenshot a map and circle or highlight details for whomever I am sending it to. I tried to sign up, but their website says they are preparing an updated version for 2014. I'm on the mailing list if they get round to publishing it! I'll let y'all know.