Years ago I was opening a bank account in Chicago. She asked me for my mother's maiden name. I must have looked at her funny because she explained it was for security. I laughed. I told her that wouldn't work where I was from. I knew everyone's mother's maiden name. She said I'd be surprised at the number of people who didn't know their own mother's maiden name.
That was long before the internet. About 25 years ago I was on Compuserve and people traded genealogy in the Roots Forum run by Dick Eastman. I was careful to end mine at my great grandparents. Later I picked a date of 1900 and gave no information after that. When ask my mother's maiden name for "security" purposes I created a maiden name she would not recognize. Early on that was a problem because I was rarely asked and I would forget. So I changed her maiden name to something I could remember. She's had that same maiden name since.
About 15 years ago signing up for the first free web mail account [I still have it] it asked me my birth date. I didn't figure it was any of their business so I created one. They only want to know if I'm adult and the birth date I created indicated I was. Wouldn't you know I forgot the password and they asked my birth date to retrieve it. Fortunately she also gave me a password hint and I got it. [Yes, back then you talked to a real live human.] So I created a "permanent" birth date for myself. Naturally I made myself younger and with a more interesting birth date.
I still don't put out the details in my genealogy but it wouldn't take too much to figure it out from reading blogs, mailing list posts, etc. And a couple of people have posted my lineage all the way to me.
Security questions have gotten more detailed. I can't answer them. What street did you live on when you were 6? When I was 6 I might not have known what a "street" was. Certainly the roads were not named in the country. So I pick the genealogy question and use my creative facts.
The criminal who uses my mother's maiden name, my birth date, etc. from my online genealogy is in for a big surprise.