I mean, they asked for my login information to search my Facebook. I declined, but they still ran through all the public information and connections I had.

Later, I worked on some of their creepy background checking software. Essentially, without the login, they can still get all the information about me they need.

If this happens to anyone else, just quote Facebook's Statement of Rights and Responsibilities. https://www.facebook.com/terms

4. You will not share your password (or in the case of developers, your secret key), let anyone else access your account, or do anything else that might jeopardize the security of your account.

It's not iron-clad, but it may dissuade the nosy.

This will of course depend on your existing privacy settings.

Those change over time so Facebook exposes more of your information you meant to keep hidden.

I have to check my Facebook every few months to find out what sort of content is now visible to a new group of people that was hidden by me in the past.

Really? I haven't found that to be the case. Everything that I've set to specific privacy groups (me only, friends, etc) has stayed the same.