In a comment on one of my previous posts, Carmen Holotescu provided a link to an article called Can We Use Twitter for Educational Activities? that she co-authored with Gabriela Grosseck. The article is a good overview of microblogging in education. It also details some of the features that are lacking in Twitter but which would make the site more user-friendly.
For example, it would be helpful to be able to use tags. As interesting as it might be to learn about daily activities of some Twitterers, I would prefer to only read their tweets on professional issues. I just don't have time for more than that.
Another feature missing on Twitter is the ability to create user groups with notes visible only for members. I believe that some of these features may be available in Edmodo, but I'll have to check. Carmen is advocating the use of a new Romanian microblogging site called cirip that incluldes some of these features along with some others.
I'm going to look more at Edmodo today, but it seems to be much more than just a microblogging site. So if schools don't block it , the site may have lots of potential for classroom use.