If u go to ur contacts (assuming u already install the facebook app)
click options
then click get friends( a screen with facebook and add account should pop up)
Click facebook and that should solve ur problems.
The only draw back is that all ur friends from facebook will be in ur phone. Pretty crazy if u have alot of friends. Plus u got to link some of them to the phone or google entries that was already on the phone. Mind u even facebook friends that dont have contact numbers will be added. And u might have to delete alot of them
Its up to u friend...