Social Network

I have taken a break from teaching to raise 3 boys.  I will be teaching high school mathematics this fall. I could really use some good ideas (websites) as to how to go about using the social network in my classroom to communicate to my students.  I want it to be safe and somewhat private if some students who have a hard time asking questions in class would be more willing to ask me online if they knew not everyone was going to see it.

Any ideas???

Michelle