Edmodo is a wonderful classroom tool that can be easily compared to a Facebook type social site where only you and your class can gain access and post.
On this site you can give assignments, take polls, assign quizzes, send alerts, and just interact with your class through posts.
I have used this site for two years now and I absolutely love it. The kids get a big kick out of it and love that they can communicate with one another and myself outside of school hours.
There are apps for this program as well. Available on iOS and Android.