Tom Scott developed a site he called Evil that displays random Facebook users with their phone numbers using standard Facebook API. The phone numbers are public info on Facebook, although their owners probably thought it is secured.
Phone numbers can be obtained using means such as white pages, but Evil shows that your Facebook info is probably public to the world even though it might not be what you want. And most Facebook users either assume all things private by default or do not setup the privacy settings in Facebook to their liking, or simply do not bother about it.
[Evil by Tom Scott](http://www.tomscott.com/evil/)