Why do people even see Facebook integration as a bad thing?

While you need to sign in with a real Facebook account, which everybody has at this point and there is no reason not to just make one, it is not as though it will be displayed to the public if you don't want it to be.

Of course you will be able to hide your real details in game.

It is not as though Facebook is doing anything malicious with your data anyway. Targeted advertising is valuable because it works. Who wouldn't want ads that are more revelant to their interests?