I didn't suggest that you could find someone else's IP on What's my IP or similar websites, I was merely pointing out that if a third party website like that can detect my IP it must be something that's easy to do and thus it's nothing special to the evil Facebook that everyone so despises at the moment
On its own, having a person's IP address does little more than display the user's geographic location, which is fairly harmless - on its own. Things start to get interesting where people are on the move and their IP address changes. Remember that all IP addresses are logged and stored by Facebook. Such info quickly shows travel (and timing) patterns, which are useful when targetting holiday ads or related services. Moreover, most users activate (or leave activated by default) location sharing on mobile devices, which narrows down geographic locations much more accurately, sometimes to a matter of 30 metres or so with "always on" mobile internet connections. How often you login, and how long you spend on particular areas of Facebook are all modelled and stored. Ditto click-through:-
What is a Clickthrough? - Definition & Information
Take it from me that no other website on this planet comes even close to logging as much or as varied data as Facebook. Remember, EVERYTHING that you do on Facebook is stored and analysed for future research. Yes, all of it. Every. Single. Thing. Some of the data is stored at a highly granular level. Other aspects are quickly pre-processed via aggregration or ranking them in descending order. Regardless, you are being closely watched and it's all automated. Later, data mining algorithms analyse it and act accordingly. It's called customer segmentation:-
What is customer segmentation? - Definition from WhatIs.com
I know this, because I used to work inside the business - not for Facebook, but on other projects. For most naive users, a website is simply the pretty user-interface that they see in front of them. Bright colours, widgets and it's all so clicky-clicky etc. However, behind this facade there's a vast, unseen world of the MySQL or PostgreSQL databases plus XML code etc. Few people ever lift the lid to see what's behind the front end of a website. They don't care and/or don't have the skills.
Facebook isn't evil. However, it is amoral. People are the product, and Facebooks carefully studies its users as if they're bugs under a microscope. And just like those bugs on the microscope slide, most Facebook users are blissfully unaware of what's going on in the background. This is the nature of the beast.