Facebook can only reveal as much as you let it reveal. I keep mine on "lock-down", meaning that only my friends can see my posts, my info, and my pictures. If an outsider looks at my profile, all they see is my display picture, hometown/where I live/schools, and my name. Many, many people have their profiles wide open to everyone, and you can see anything you need to before actually meeting them in person.
I also post very, very rarely, like once every two weeks. And, what I do post is about my day (having a really bad day or good day, or a weird observation).
What you "like" does show a lot about someone however. I have a long list of "likes".
Be aware of what you say anytime that your being recorded, either when you write/type or are on film or audio.