My Facebook profile link is in the form https://facebook.com/john.smith where John Smith is my display name. The email however which I use to log in is email@example.com
My mom's Facebook profile page is https://facebook.com/maryp while her display name is Mary Poppins, while the email she uses to log in is firstname.lastname@example.org
Furthermore, I have friend, when I went to his profile page, address was https://www.facebook.com/profile.php?id=100000077777777
How come Facebook uses different ways to form profile URL? Firstly, user public name, secondly, user email (which is not secure at all), and thirdly, user id? Where this can be changed?