I have created a facebook group each for both courses I represent. It was relatively easy to find everyone on facebook as most of them were already in other law related groups or friends with me. To ensure that absolutely everyone can participate I asked the class beforehand however, if they would be fine with a facebook group and invited everyone to join as soon as no objections were made.
It helped enormously in gathering feedback and in discussing class related topics because now everyone participates in these discussions. A mailinglist would have been not that good, because it would spam people's mail folders and would only serve as information instrument. The facebook group doesn't "annoy" anyone and can also be used as information sharing instrument, while having the benefit of making discussion's possible. Of course I also gather feedback outside of facebook, because I don't want to risk missing something should someone not want to write it publicly in the group. For the vast majority however it is not an issue and it makes my life a bit easier because I now can also see how many people agree with someone's point or share criticism.
I have the feeling that it also helped developing a community feeling because now we all have more contact with each other. Before we only saw each other once a week and had hardly contact outside of the classroom. This changed radically now and people feel more connected, we even go out together now as a class. I am more accessible because people have now my private contact details and are apparently more willing to drop me a facebook message than they were willing before to write me an email.