First of all, it's totally free. It is also super easy to get your students in. You make your account and then create the class. The website will assign you a group code. When your students have their tech in front of them, have them go to "get Secure Student Account". They put their group code in, their name and create a password. No email required. It automatically adds them to your class. It's completely secure.
So, the kids call it "Facebook for kids". They can easily send each other messages, privately or to the whole group. But here is one way I used it today... our school is a global studies school. We study our continents through Project Based Learning. For my class, I like students to think of their own driving questions. However, these are new fourth graders (essentially still third graders) and don't have much practice with project based learning or technology. They voted to start their inquiry with France. Today, they used Edmodo and Google to spark their thinking. I taught them how to have two open tabs: one for Google and one for Edmodo. They used the Google tab for research on whatever topic they were curious about. The Edmodo tab was for collaboration. They asked questions about France. They were able to add pictures and links very easily. Tomorrow, I'm going to compile the questions and we're going to separate them into "good" and "maybe not" questions for deep inquiry. Here is an example of what they did:
I've also used Edmodo for small group work and to send tiered assignments. My kids love the apps. Subtext is a great app and is something I'll write about another time.