I’m risking of stating the obvious with this one, but I’d bet my shirt many of your were unaware of this little trick. As you know, Facebook integration in iOS 6 and OS X Mountain Lion can automagically update photos and user names for all matching contacts. What you may not have known is that Apple only needs a person’s phone number to pull this data.
So if a girl you met at a bar last night left you just her phone number, you can easily get her Facebook profile picture to appear on your iPhone when she happens to call you - even if you’re not friends with her on Facebook and even if her profile is hidden on Facebook. Plus, you'll get her full name in Contacts. How cool is that?