In case you lost your mobile phone, and eventually the contacts too, you need not worry unless you are a regular on Facebook. By regular I mean your network on FB is kind of vast and is outgoing. There are 50% chances that your friends will have entered some valid Phone number. And you can access that List of your friends` phone numbers on FaceBook at http://www.facebook.com/mobile/?phonebook.
Nice, handy feature indeed and believe me, I found this page, The Facebook Phonebook, while browsing facebook from my Mobile Phone. The implementation of this PhoneBook feature is neat. One FBML query, into the Fdatabase for a variable that deals with the phone numbers. A read through this should have you going better ;). The screen shot below should give you and idea of wat the thing looks like.