Etiquette is important on the internet and especially in real life. This is something I address on a pretty regular basis, or maybe it just seems that way. So important in a room with so many tempting things to touch.
I would address the dangers of sharing too much personal information as well for all forms of communication.
The importance of not believing everything that is on the Internet is important, as well as the way citing sources and not plagiarizing work, are other topics to discuss as the need arises.
As with other subjects of importance, repetition and stating an idea many different ways is key to teaching these important concepts.