I've been using blogs with students for almost 10 years now, and the results have been erratic that I stopped doing it. One specific reason was getting students to comment on each other's post, and I had better results using online discussions when I required them to respond to the previous poster and post a question for the next one.
Then I tried Ning last year with the yearbook class, and also introduced it to the high school English teacher. He was happy to have this option, especially when they started reviewing literature. Ning offers features that allow students to interact with each other, that our Edline site does not do (it is primarily a course distribution and management resource). I still didn't think I was fully utilizing Ning's features with my own yearbook students, they were only posting stories and photos but hardly interacting. They had Facebook for that!
Things changed this school year when I started teaching Middle School. I initially set up the site so the newsletter students can post suggestions. When the middle school introduced the intramural it was a great way to get students to post ideas and comments about the games. The newsletter class got to use Poll Daddy for polls to add to their stories. Photos of games and other school events like dances were uploaded, and this was my "pull" to get students to visit the site. But what really got them was the Buddy Poke application. It was fun and addicting for them. Soon enough I had students from other classes begging me to add them as members.And it was great for the Computer Literacy class, because of our cyberbullying unit.
Right now I'm having the Digital Media class post blogs about Anime Studio. They were required to post the three coolest tricks they learned, and give the step by step instructions for doing it so everyone can learn from each other.