The source of the rumors is a misinterpretation of a feature called "Contacts." When you download Facebook's mobile app, this feature syncs your phone's address book with your profile. From then on you can access all of the numbers in your phone from your Facebook profile.
What has made some users uneasy is this: Contacts uses your mobile phone contacts to match your friends' Facebook profiles with their numbers. If a friend hasn't included her number on her Facebook profile, it looks as though Facebook has just given you her number when in reality it came from your own phone.
The feature also uses the numbers in your phone's contact list to search for potential friends on Facebook. If it finds somebody who has posted a number on Facebook that matches one in your phone book, it will suggest that you add them as a friend.
To see the feature in action on your own profile, go to the Accounts tab at the upper right-hand corner, select "edit friends," and then choose "contacts" from the menu on the left.
A viral Facebook post warned users that Facebook also posts the numbers it finds on your phone to other users' Contacts tabs. This is not the case, the company said in a post on its wall late Wednesday.
"Our Contacts list, formerly called Phonebook, has existed for a long time," the statement says. "The phone numbers listed there were either added by your friends themselves and made visible to you, or you have previously synced your phone contacts with Facebook. Just like on your phone, only you can see these numbers."
While Facebook says it is not sharing phone numbers collected from mobile phones, its ability to populate them in this way is still disturbing to some users, who left a total of about 9,000 comments on the message.
"You should have to opt INTO this feature, rather than have it happen automatically and have to uninstall it," wrote user Heather Hollowell.