You can say anything. It doesn't matter. Nobody will get mad who works with Facebook or kick you off of it. But, be aware: If you're chatting with somebody on there, and you say something mean to them, they can report you. So just be careful with what you say.
De Facebook
I think "on facebook" is right, for it's a website.
Bonjour Facebook.
Facebook es aburrido
inscribete con Facebook
Está em Facebook
Willkommen bei Facebook.
bienvenue sur Facebook
Estoy en facebook.
Agregame en Facebook
Most likely because you are.
Buenos Dias Facebook