Edmodo.com is a great way to communicate with your students online in a secure way, and best of all...IT'S FREE!!!!. The platform was created by educators and made for the classroom. Some of the features of Edmodo include being able to post notes to students (they can respond back to you or the whole class), attaching files to your post, or posting links.
The way it works is pretty simple. As the teacher you register for an account. Once your account is created you can create your classes. This could be just one class (ex. Mrs. Burnett's) or multiple (ex. Math p.1, Math p.2...). With each class that you create you get a code. Students then sign up for their own accounts then enter the code for your class. It's good to note that students can have multiple classes associated with their account (Parents can also sign up for accounts so that they can view their child's account).
Once everything is created you can get started posting to your class. As the teacher it is important that you have conversations with your students before they start using Edmodo about appropriate academic behavior online. I would also make sure and mention to your students that what they post can either be a direct message to you the teacher, or to the whole class. There's no in between. You will have documentation for everything that students post as they will have to be logged in to access your class, and students can attach assignments to their posts to turn into you as well.
I think that this is a fantastic tool that could be a great use of social media with your classroom!