Way back when, Facebook asked its members to provide a phone number that could be used in emergencies by Facebook. Given the number of suicides prevented, elderly people rescued, etc., that seemed a commendable purpose. But one of the promises was that the security number would be kept private. Surprise, surprise — Facebook allows the outside world to search that number so anyone can find you.
The smallest group of people to whom you can limit access is your friends. This is simply wrong. Facebook has been made aware of objections and has not done a thing to change its setting.