The three things I want my students to know are:
Internet safety - Do not accept all friend requests, and do not have 819 friends! Some of those are going to be creepy, scary people. Only have friends you know in real life. When I discussed this with my students at the beginning of school, they admitted to accepting people they don't know. Then they told me some scary stories. Some students are being too polite, and not wanting to "hurt" the other person's feelings. We had a great discussion about listening to your instinct, and walking away.
Deep web searches - Like the sources Siri (the iPhone voice) uses to search with. She does not use Google!
Etiquette - Student's generally are very open to learning about this.
I used the CyberSmart e-book collection this year to teach Digital Citizenship. It's on the Library Resources page. It has web content, and handouts. It also has a rubric. We had some very interesting classroom discussions.