I wonder if all my FB friends know that their phone numbers--and possibly all the numbers stored in their cell phone--are publicly displayed on their profile page, and possibly even alongside their name in their friend's contacts.
This article explained it pretty well:
Facebook has a history of fishy privacy practices, so we're not surprised at its latest grab for user information. If you use a Facebook app on your cell phone, have provided your phone number to it for "increased security," or entered it into your profile under certain sharing settings, your data could be at risk.
Rumors abound that Facebook is now making public contact lists from smartphones that use Facebook Mobile. Although Facebook denies that this is true, a longstanding feature that syncs contacts from your phone to your Facebook account has been discovered by many. This means that Facebook has probably stored every number in your phone on a server somewhere.The main thing that is for sure happening is that when people enter their phone number '...for increased security' the number is showing up in your profile. When you think of how casually we friend people we've just met, that's a bit disconcerting, IMHO.
As one guy wrote about this, even though I actually know a few of the people I'm friends with, I still don't think I'd hand out my phone number to hundreds of random strangers in a bar. Would you?
To remove you number go to profile > edit profile > contact information and delete it.
While you're at it, you might not want all 500 of your super-close friends knowing your address either.
But that's just me. I'm kinda paranoid like that, I guess.