iOS 6 is simply mirroring a feature available on Facebook’s website. The feature enables users to search for emails and phone numbers on Facebook, and users can allow their email and phone number to be searchable by editing the “How you connect” section and by setting the “Who can look you up using the email address or phone number you provided” to friends.
This feature could come in handy if you are trying to fill out contact information without all the necessary data. For example: If you only had the phone number of a person you met at a party, you could easily gain the rest of his or her details. If users do not want the information shared publicly, and therefore obtainable by Facebook integration within iOS 6, they can simply make a change to the setting.
If this concerns you, head over here to edit your Facebook prefs.