Add mobile number to Facebook contact?
From your contact list, select the Facebook contact that you would like to edit.  Click the menu button and choose Edit Contact.  You'll see an indication that the user is a Facebook contact and may not be edited.  Now click the Menu button again and choose New contact.  This will create a new contact in your address book and merge the Facebook profile information with that contact.  You may now edit the contact to have additional details, such as a phone number.