This video is geared towards elementary school students but it offers insight into the difference between formal and informal language and why it is important to know when to use either.
This video offers some clear principles for participating online.
Being online brings additional powers and responsibilities than in the in-person classroom. Your words have stronger effect than ever because they are published and can be reproduced over and over again.
When engaging in Discussion Boards or posting messages for your whole class to see, doing the above makes sure that the environment stays respectful and on-task for everyone.