A Facebook user backlash is not an uncommon thing, and Wednesday turned out to be one of those days for the social networking site. It turns out some portion of Facebook's 750 million user base cottoned on to the site's practice of syncing smartphone address books with user contact lists—and really didn't like the implications.

So what's really going on? It turns out that Facebook users who sync their phone contacts with the site see those numbers pop up in their contact lists. In addition, any contact details a Facebook friend (or even a non-friend, but more on that later) has made visible publicly will appear on users' contact lists. Finally, contact details revealed by Facebook friends to other friends but not to the public also appear.

If you're a Facebook user, to see how that works, click on the Account button on the upper right side of your Facebook page, click on Edit Friends on the scroll-down menu, and then click on Contacts in the left hand column that appears on the page (see example below right).

What you'll see is a list of your Facebook friends with associated phone numbers. But who else can see those numbers?

Wednesday's kerfuffle over phone syncing was the result of Facebook users' fears that all those phone numbers might be seen by others and used for any number of nefarious or at least mildly irritating purposes.

Rumors claiming that your phone contacts are visible to everyone on Facebook are false," the company said on its own official Facebook page. "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."

Facebook also says there is now a permission screen that pops up before a user syncs a phone with the site, and offers up this tool for removing unwanted address book uploads after the fact.

Meanwhile, there was a similar outcry back in January when Facebook rolled out address book syncing for the Apple iPhone.

But if the issue is an old one and the worst fears of those making noise are unfounded, some of the more credible complaints simply haven't been addressed by Facebook at all, say some observers.

Users commenting on Facebook's own explanation post have said they weren't given enough information about syncing up their phones to know they'd essentially be handing over their entire address book to the site.

Some have also reported seeing contacts listed with phone numbers on their contact lists who aren't even their Facebook friends at all, leading them to wonder if their own contact details are populating strangers' contact lists. (For a reminder of what you may be revealing to the world at large on Facebook if you aren't diligent about maintaining strict privacy settings, simply visit this site.)

MORE: What's Facebook doing with all this data, anyway?