As its name suggests, FacebookSync is a tiny freeware app from FSB Software that - wait for it - syncs your friends' info from Facebook to Mac OS X's Address Book. Upon running it, a window much like the one you see above will appear for every contact that either doesn't have an Address Book entry at all, or is missing information that could be pulled down from Facebook. It can even pull down images in case your Address Book contacts don't have those either, which is handy since apps like Mail, iChat, Growl + Gmail and Adium can use these pics to help identify these contacts.
Whilst trying this out, FacebookSync seems to work pretty well, though I admittedly don't spend much time on it and I only have a dozen or so friends. Still, it added the information I told it to (and for the record, I'm on 10.4.9 and have over 450 contacts in Address Book already), so I'll give it two thumbs up. Before running FacebookSync, however, I highly recommend you chose File > Backup Address Book in Address Book to create a clean, recent copy in case things go south. FacebookSync isn't likely to eat your cat or cause your Apple TV to implode, but having a Plan B is an absolute must when playing with software like this.