Social Media - our enemy?

I've been a head of year and I know how many problems are caused by malicious use of social media.  But social media is in itself neither good nor bad.

Can social media be used to promote learning and if so how?
How can we use social media and ensure pupils and staff stay safe?
What guidelines can we give to pupils about using social media safely?
Is it possible for us to teach pupils how to use social media well if we can't access social media in school?

http://critical-thinkers.com/2011/10/can-social-media-in-the-classroom-encourage-critical-thinking/