It has to do with your phone numbers. If you have a Facebook page, and you find your way to your "Phonebook Contacts," you'll find a list of friends, along with their phone numbers.
It looks harmless enough, except that you didn't knowingly put them there. Either your friends posted their numbers on their own pages -- or you added them by synchronizing Facebook with your smartphone. Did you know you were importing phone numbers when you did that?
The feature is not new, but messages about it have spread at Internet warp speed among Facebook users this week. Despite the company's denials, thousands of users have come to believe, in just the last few days, that their phone numbers are right out there in public, for all the world to see.
"I never included my personal phone number on my account," said one fairly typical message. "However, I found that phone numbers from my phone were uploaded to my FB account. I did not ask for this and hope I fixed it on my phone."
"I didn't know this was happening until today," wrote another user. "There are numbers there from my phone that shouldnt be."
It wouldn't be a big issue if Facebook itself were not so big. As of July it claimed 750 million active users. If it were a country, it would be the third most populous in the world, behind China and India. If your friends' numbers were actually made public, people worldwide could find those otherwise private phone numbers with little trouble.
"We are not able to pinpoint when, where or how this meme started," said a Facebook spokesperson in an email to ABC News. "We became aware of the confusion earlier this week and immediately started informing press and people on Facebook that this is not a new feature but one that has existed for some time."
The company added an assurance at the top of people's Phonebook Contacts page: "Only you can see your contacts." And it posted a message on its own Facebook page:
"Rumors claiming that your phone contacts are visible to everyone on Facebook are false," the company said. "Our Contacts list, formerly called Phonebook, has existed for a long time. 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."
Did we say Facebook is big? By Friday afternoon, nearly 35,000 people had "liked" the post -- and another 17,000 had replied with comments. Some were gibberish, but many were vociferous complaints.
"I don't understand what business any online company has snatching private contact information from my phone," wrote one user.
"Where was the request to grant permission to sync those phone numbers with Facebook?" wrote another. "Disgusting behaviour, truly disgusting."
There is a way to turn off the feature, which you can find at https://www.facebook.com/contact_importer/remove_uploads.php?r=%2Fphonebook.
But to many users, the damage was done. Users and privacy advocates have complained in the past that Facebook introduced features and assumed users wanted to enable them, forcing them to feel their way through their account settings and opt out.
"I think you need to notify us of these changes... period," wrote a user this morning. "It is an invasion of privacy, you should have permission to do this. What if someones profile gets hacked or phished?"
"For shame Facebook," wrote another. "Please ask in future."