It is used to gain as much information as it can, and then sell it: to advertisers, employers, creditors, governments, ad nauseum.
Integrating it into any Apple device will key my exit from Apple's technologies.
As with Twitter, Facebook would be completely optional. To say you'd never buy Apple products again because they offer an OPTIONAL service integrated in the OS is quite childish.
To this day, I haven't used Twitter on either of my iDevices. In fact, I deleted my account a couple of years ago. I don't see the point of one sentence garble, and I was getting tired of receiving requests for people to follow me that I had no clue who they were.
Facebook, on the other hand, is quite different. It's been a great way to connect with old friends and distant relatives that I haven't seen or talked to in many years. I have everything on my account set to private, so it isn't widely available for the unknown world to see.