You could say "Hola, esta es su amiga hacking su página Facebook." Many computer terms go directly to Spanish from English, especially slang terms like "hacking".
Agregame en Facebook
Estoy en facebook.
Facebook es aburrido
inscribete con Facebook
Tienes un accuenta en Facebook?
Buenos Dias Facebook
To say hello friend in Spanish you can say "hola amigo".
You could say "¿Por qué estás en facebook?"
If they say 'Inbox me!' or something like that they mean they want you to add them as a friend on facebook.
Estas conectado(for guys), conectada(for girls) en facebook?
bueno no mis amigos del facebook
my friend = mi amigo