This could be a great way to communicate with students in an online class by using cell phones to remind them of assignments/upcoming due dates/tests. It is free and students and teachers don't share their cell numbers.
My wife introduced me to is blog. It has lots of free technology ideas for teachers. Just looking around I can see many online resources that I could use if I start teaching again. And someone else is keeping it up-to-date for me. That is even better!
If you don't give students appropriate and timely feedback, how will they know what to do? This is important in a face-to-face class and probably more important with online classes when you don't see the teacher each day.