Facebook and Twitter for the classroom in one
I love using this with my classes. This allows me to set assigment (groups and individual), give quizzes and homework. If I need students to view or read something before class, you can post in advance. My student loves Edmodo. When they forget the homework or the details about homework and projects, they can send a post to the class and someone will reply. The teacher might reply.
How I Use It
Teacher sign up, then set up a class. Once you set up a class they will give you a joined code. Have student sign in as students and give them the code.