If you've ever used Facebook integration in iOS 6, you may have noticed iOS lets you grant the Contact app access your Facebook account in Settings > Facebook. Enabling this feature creates a new 'Facebook' group in Contacts which contains entries for all of the people you're friends with on Facebook. According to a thread over at Apple's support forums, a number of people are experiencing an issue where some of the entries are dissapearing from the Facebook group in iOS Contacts.
If you have noticed similar symptoms, don't panic - it's a feature rather than a bug...