A spokeswoman for the social network told the Daily Mail: "Only you can see your Phonebook on Facebook; it can't be seen by anyone else. This is the same as online phonebooks or email contact lists many people already have."
"People also have the ability to control who sees their contact information through their privacy settings – this includes the ability to make your mobile number visible to only a few select friends or of course you don't have to include one at all."
Concerns had been raised by Charles Arthur, a Guardian blogger under the headline: ' Is your private phone number on Facebook? Probably. And so are your friends', and reported by various leading tech sites including ElectricPig.co.uk.
"Facebook never shares personally identifiable information with third parties - advertisers are only given anonymised and aggregated data," Facebook added.
"Facebook is a free service and something that many people find adds value to their day-to-day lives. As with any service, users do need to invest some time in order to use it properly and we encourage people to use their privacy settings to do this."
ElectricPig.co.uk wrote in an update: "The current version of the Facebook iPhone app has a warning for syncing users. Those who synced with an old version won't have seen the warnings though, and we're still left wondering where some numbers in our Facebook address book have come from, and exactly what usage rights Facebook has over the content."
"There's a large remove data button in the Facebook app when you have sync turned on but that appears to just delete Facebook contact info from the phone. Removing the details from the web which have been synced isn't simply explained. It seems you need to turn off the sync function in the iPhone app and visit this link." This feature appears to breech Apple's App Store Review guidelines (PDF link).
Charles Arthur has meanwhile updated his original post, with more details and Guardian readers comments. He also links to 'This is Evil by Tom Scott' a site which randomly displays the private phone numbers of unsuspecting Facebook users.
"There are uncountable numbers of groups on Facebook called 'lost my phone!!!!! need ur numbers!!!!!' or something like that. Most of them are marked as 'public', or 'visible to everyone'. A lot of folks don't understand what that means in Facebook's context — to Facebook, 'everyone' means everyone in the world, whether they're a Facebook member or not. That includes automated programs like Evil, as well as search engines," Tom Scott notes in the introduction.
See also: Facebook overhauls its Groups feature