I've seen one that asks the last 3 digits of your phone number. Not sure how useful that one is but it could probably positively ID someone with a reasonably unique name.

My feed has become littered with these things because my friends are replying to them. Which encourages me (or, rather, the hypothetical FB user) to reply to my friend(s).

Added to correlational hints as to my location, it would clarify six of ten digits of my phone number.

There are probably other clever ways to skeeve the other four, and I cannot say for certain if some digits of those other four are situationally impossible. Certainly some combinations likely are, like 0911, and 0000.

If you already have a list of possible real numbers 6 of 10 sounds like it could narrow it down a lot.