I usually hate these kinds of infographics, but this one does summarize some basic guidelines. Of course, the main reason to learn the rules is so you can break them at the appropriate times. By the way, are these long tall infographics taught in high school or “social media 101” now or something? I get emailed 5-10 of these a day. Bizarre. Anyways, if you have some need for them, there’s a large collection over at Pinfographics.