Edmodo is one of the tools I have used with classes. I helped one of our Social Studies teachers get her students started with Edmodo and they've been rockin' and rollin' with it ever since! Looks like there have been some quizzes lately to take online.
The nice thing about Edmodo is that engages all students in an educational facebook sort of way. So you are hearing the voices of everyone, not just the most vocal, outspoken students. It's easy to set up online discussions, homework assignments, surveys and quizzes that students can access anytime they have internet access. There's even an iPhone app for that!
Twitter is another tool that can be of great value in the classroom. I personally use it to keep up with the latest educational news and research. Here's a sample of organizations I follow.
Twitter can be used in the classroom to get real time information about current events. It also offers the opportunity to connect with people from all over the world. It might be something you could challenge students to use to seek out opposing viewpoints etc.