Contacts (previously Address Book) has long allowed you to select one or more cards and choose Card > Merge Selected Cards to help you clean up duplicate contact information (Card > Look for Duplicates is also a great way to make Contacts do a big cleanup job for you). But in Mountain Lion, Apple has expanded this feature to account for optionally syncing your contacts with your Facebook account.
If you add your Facebook account to System Preferences > Mail, Contacts, Calendars and enable contact sync, any friends who share personal information with you will now appear in Contacts and automatically update if, for example, they change addresses, phone numbers, or email addresses. It’s like having a shared rolodex that automatically updates itself—I love it.
Once you enable Facebook contact sync, you may find that some friends have separate cards (the one you added previously and their Facebook card), usually because they used the full version of their first name or their middle initial on Facebook.
Contact cards from Facebook are not editable, they’re read-only. If you select a card you added for a contact and their new Facebook card, the Card menu Merge option will change to “Link Selected Cards.” Visually, this will combine the entries and their details so you have just one card, but if you click the Edit button you can see which details were added from the Facebook entry.
A word of caution if you try this: I’ve been doing it periodically by cleaning up my contacts since I would rather let this info automatically update from Facebook, and I’ve been pretty happy. But as far as I can tell, there’s no way to un-link cards once you do this, and if you delete the card, you are only deleting the local details or the iCloud-synced card you previously had on record. To get rid of a Facebook-synced person from Contacts, you need to unfriend them on Facebook.