- FB friends with no contact info - will not appear in iOS
- FB friends with contact info, that you do not have as a contact in iOS - will appear in iOS
- FB friends with contact info, that you do have as a contact in iOS - will appear, may not be linked properly, can be linked manually
I also noticed some contacts in iOS have an outdated FB picture (and no "F" on the picture). I suspect these contacts were once synced, then removed their contact details so the sync is broken. But the old picture remains.
I can't remember if I was able to sync friends with no contact details before. If this were the case, maybe they didn't have contact details all along, and the new rule broke the sync.