And it didn't have pictures next to the number making it easy to target certain demographics.

yup, having a photo associated with a phone number is more concerning than a name. In this case you can actually say, Think of all the children!

Or an associated set of "Like"s - reasonable indicators of a person's outlook.

Numbers of all the hot girls I could never get in high school :-( where was Facebook back then!

If you "could never get" a girl's number, what would be the point in having it? The point isn't only to have someone's number, but for her to actually talk to you.

Do phone books have cell phone numbers now? (I haven't picked one up in ages) I suspect the majority of FB users are posting their cell numbers.

No, not everyone's phone number. You could opt to have an "unlisted" number, and it was easy to check whether or not the book was revealing your number to the world.

You can (easily) do that on Facebook, too. It even lets you view your profile from other perspectives. And it's even easier than calling the phone company to ask for an unlisted number.

This uses the graph API to search through "lost my phone / need ur digitz" groups. Of course, human error is the best "in" for a hacker, and one of the greatest places to look for vulnerabilities is rare combinations of use cases. (People usually check privacy settings, but NOT for "groups"). Mobile apps are another great place to check for vulnerabilities, because "mobile" is a parameter that changes up the combination and introduces the possibility for (privacy, in the case of Facebook) vulnerabilities.