Much of the time your phone contacts and your Facebook contacts are going to be one and the same, or at least have quite a bit of crossover.
iOS allows you to match those contacts together. To do so, head to the Settings app on iOS, then click on the Facebook button. There, if you have your account connected, you can click on the “Update Contacts” button and then sync them together.
The syncing procedure also adds phone numbers to your Facebook contacts.