Social Media in High Schools

As part of a project for a Learning Technologies graduate class, I'm doing research to find out how teachers are using social media in the classroom.  I currently have a closed facebook group and a twitter account that my students follow for updates, conversation, links, etc.  The twitter feed is great for quick and short info, but the facebook group has been more helpful since it's a two way street. 

Neither of the accounts require me to be friends with or follow students.  In twitter, the students follow my profile and we use a common hashtag to communicate when necessary.  The facebook group is a closed group that students request to join and I approve.  I receive notifications every time there is activity on the page.  Part of the trouble is getting students to join and use the social media for educational purposes.  This seems to be new territory and they hesitate to participate. 

A few questions:

1. How are you using social media with your high school classes?
2. Do students actively participate or do they simply receive information from the instruction? 
3. What have you done to involve students and encourage active participation?

Any experience you can share would be welcome!