I remember a bunch of friends being upset about this. Apparently it's an outrage that your friends can do this, and perfectly fine that facebook probably does it all the time.
The scope of behavior Facebook is capable of obtaining is probably beyond comprehension for most users. Having a slice of that scope opened up and dissected publicly is quite creepy; despite intellectually understanding how much data Facebook has about them, actually seeing that data in use creates a much more visceral reaction that simply intellect can not create.
Having somebody intentionally look into your life, individually, because of some possibly perverse or unwanted interest in you is extremely creepy.
Having a large company store some data you generated in a server farm somewhere while a mindless algorithm does some math with that data to shuffle a few ads around for you to see is utterly banal and not creepy.
> Having a large company store some data you generated in a server farm somewhere
... and make it a available for somebody to intentionally look into your life, individually, because of some possibly perverse or unwanted interest in you