Common internet rules

Are there any common rules about what you can and cannot write?

On facebook, for instance, there are some terms and data policy you have to follow, but because of the length of the rules, there aren't that many people who read them, and therefore no one really knows what they're allowed to do and what they're not allowed to do on facebook.

Most people use common sense about what you write, and what you DON'T.

But like in real life, there are a lot of people who doesn't use their common sense.