iOS 6 features tighter integration with Facebook, including adding your Facebook friends to your contacts list, not to mention adding their birthdays to your calendar. Learn how to loosen Facebook's grip on your contacts and calendar.
Perhaps I'm slow, but just I noticed that all of my Facebook friends are now littered throughout my iPhone's contacts list. This change is a result of iOS 6's tighter integration with Facebook, so I've been living with it for a few months now. (In my defense, allow me to state that for most calls, I use the Favorites list, where Facebook contacts don't show up instead of the All Contacts list, where they do.)
If you have been living with a contacts list inundated with hundreds of Facebook friends, there is an easy fix. Head to Settings and scroll down until you find Facebook. For the Facebook setting, you'll see a section titled Allow These Apps to Use Your Account. Tap the slider to Off for Contacts. Now, head back to the All Contacts list and you'll see your list returned to its state prior to iOS 6 -- listing only the contacts you entered.
Not allowing Facebook to access your contacts also has another effect: it cleans up the calendar app by removing birthday reminders for your Facebook friends. And if you'd like to keep Facebook events from showing up on your calendar, turn the Calendar slider to Off in Settings > Facebook.
For more, I would direct you to Jason Cipriani's post on Understanding Facebook integration on iOS 6.