I've been using Blogs for about 4 or 5 years now via Housman's After School Programs. I thought students might be more interested in writing if they didn't have to struggle with pencil and paper. Some of the students proved to be amazing writers and others were more interested in finding widgets rather than putting fingers to keyboard. Originally I let them chose a topic of their choice but after the umpteenth post about Justin Bieber and WWF I was forced to impose a bit more structure. I found I had to specify how many sentences qualified as a post. Oddly I found myself doing the very same thing for teachers when they tackled the 11 Tools.
Here's a link to one of my favorite student blogs - the author is going into the 5th grade and he's been blogging for 2 years. As you can tell he loves making Animotos. I have 102 student blogs on my Housman Blogger account.
The library's also had a delicious account since I first did the 23 Things. It's proved to be invaluable since elementary age children can't type long URLs. Diigo wasn't around back then but it appears to be superior and I've migrated the Housman bookmarks to Diigo while we decide which one we want to keep updated.