Edmodo looks like a social network like 'Facebook'. It even works a lot like a Facebook. But in Edmodo teachers can start online discussions, teaches can create teacher made quizzes, you can even assess on separate Common Core Standards.
Students make webpages that can have several creators at once. Very easy to use. Luckey has used this with 4th and 5th graders, They have an education side that allows you to setup sites for students. Teachers have to sign up as user and add students.