I think a good lesson about social media etiquette and netiquette would be to start off with what one could say about or to another student online and positive ways in which one can convey their ideas and thoughts.
This is an example of how teachers use paper blogging to teach students quality commenting skills. You can read more about paper blogging here - http://tvstechtips.edublogs.org/2013/09/09/when-paper-blogging-trumps-the-computer/
Comprehensive post on what you need to know about images, copyright, creative commons and using images online.
Student blogs, digital portfolios, blogfolios. Whatever you call them, this comprehensive resource gives you lots of ideas about how teachers are helping their students to create their own online space.