On school resource officer (an actual police officer) is available to come in and talk about the impact of internet communication. I also have a few incidents from past students that I can share.
a student that had the FBI visit his home because of the message he sent to the white house.
our school district was contacted by their ISP who had been contacted by an European ISP provided regarding the content one of our students posted to a message board.
There is also the recent example of the girl who through social media - help to push her boyfriend to suicide. NYTimes article in the last week or so. We also (real close to home - district next door to us) had the Marysville-Pilchuck shooting. Where communication through social media was a part of things.
I would be happy to write out longer descriptions of 1 and 2 (I also have a 3rd and 4th example) for any teachers that might like to use them in their classroom.