Q: In iOS 6 (and Mac OS X “Mountain Lion,” for that matter), what happens to my on-device/iCloud Contacts and Calendars after signing into my Facebook account via Settings? Why does signing in uncontrollably default to allowing Facebook access to my Contact and Calendar access, yet immediately offer the ability to deny (turn “off”) this access?
A: Facebook “Contact” and “Calendar” access on iOS is actually intended to allow the Facebook integration to create its own Contact list and Calendars in the appropriate sections; it does not automatically provide Facebook with access to the information that is already stored in your Contacts or Calendars, although once you’ve granted access, manual actions you specifically take such as tapping the Update All Contacts button in the Facebook settings will upload at least some of your contact information to Facebook, as noted at the bottom of the screen.
Otherwise, the Contacts option in the Facebook settings simply creates a new section in your Contacts app to list all of your contacts from Facebook as a separate virtual address book. You can either view all of your contacts on a single list, or tap the “Groups” button in the top-left corner to select which contact groups you would like to see. If a contact exists in more than one contact group, only a single, unified entry is shown, with a “Linked Contacts” section at the bottom providing access to each individual contact record.
Information is only aggregated for display purposes here, however, and your iCloud or local contacts are not updated unless you’ve manually used the Update All Contacts feature noted above.
Calendar integration with Facebook is similar and a bit more straightforward. Two new calendars from Facebook—Events and Birthdays—are added to your Calendars list under a separate virtual Calendar account for Facebook.
No data in your local or iCloud Calendars is modified or transmitted to Facebook, and this actually works in much the same way as iOS would if you had multiple other calendar accounts configured (e.g. iCloud, Google Calendar and/or Exchange Calendar).