Edmodo is like a Facebook for your students!
It is free to use, and it allows you to create a page for your classroom.
I use Edmodo as a writing station and post reader's response questions or social studies response questions on my wall that students answer. I also use it to assign projects to students, and then allow them to post the assignment on the Edmodo wall. There are so many features that are available with Edmodo, I can't list them all, but here is the video from their site that shows it in action. Take a look and see for yourself!
