Students should understand that once something is posted it's hard to get it back. This could lead to problems because its much harder to read between the lines on paper as opposed to reading body language from an actual person to person conversation. Its so important that you say what you mean in a clear and precise manner to avoid all of those issues.
The common sense website has a lot of great information on cyber bullying.
I would have the students research different times in which a poor job of focusing on digital citizenship caused the author a problem.
As far as approaching the issue with parents, I would discuss the issue at Meet the Teacher night so that they understand its importance.