title: Override activity performed by built-in UIActivity types
body: <p>Is it possible to override the activity that's performed by the built-in UIActivity types used by UIActivityViewController?  </p>
  
  <p>For example, I would like the Facebook activity to use my own custom view instead of the built-in one.  I know you can subclass UIActivity to make your own app-specific activities, but are you always stuck with the default behavior for the built-in ones?</p>