Re: simple answer ?
how can i use my keyboard to write farsi in face book or my emails
Have you added the keyboard to your device?
Settings > General > Keyboards > Keyboards > Add New Keyboard... > and select the keyboard that you want to add.
Then, to select the keyboard in your app (Facebook, Mail, etc), tap and hold the Globe key on the keyboard until a list of the installed keyboards appear... then select the keyboard of your choice.
EDIT: I just noticed that there is no Farsi (or Persian) keyboard option. There is an "Arabic" keyboard, but I cannot guess if that will work for you or not.