I have been using Edmodo with 5th Grade. It's a micro blog so they don't have to type too much, and it is private. Students do not have to have an email account to sign up for it. We've only been using it for a little over a month but the students seem to enjoy the interaction and even access it outside school. The moderator can delete any irrelevant comments.
I'm using blogger.com and love it! My fifth graders are "Blogging Through History." it is easy and fun. Yes they need an email address but it it is easy to go to "Design" then "Setting" then "Permissions" and make it so that only invited people may view their blogs (parents in our class, me and other students). This is my second year using blogger - AKA blogspot.com.
I have been introduced to edublogs.org in a technology class for teachers. They are blogs and the students can add eachother to read eachothers posts and comment! It is for educational purposes only and has created a safe environment that doesnt expose adult content to students!
Edmodo is also a good place to look. It is free, secure, and the students do not have to enter an email address to gain access to group projects, they only have to enter a group code. It is also easy to use and has several other features for teachers.