If you haven’t already, go and read this article by Charles Arthur in the Guardian, where he explains how Facebook apps on the Android and iPhone platforms now copy your entire phonebook back to Facebook without asking or notifying you.

So even if you have not given Facebook your number yourself, all it takes is one Facebook friend who has both your number and a smartphone, and they’ll get it anyway. Regardless of your privacy settings, they’ll then give it out to anyone else you have friended on Facebook, so long as they have managed to match it up correctly.

The privacy issues are obvious. What about the data integrity issue? The Guardian article already covers the problem of incorrect matching, which is bad enough. But what they do with people like me who are a bit slack about deleting old defunct numbers?

Say I have an old non-working number for Alice, who I haven’t been in touch with for years anyway. My friend Bob is still close to her, and so he does have her new number in his contacts list. We both log into Facebook on our phones. Facebook grabs Alice’s old number from me, which it has not seen before. Since Facebook thinks this is a new number, it helpfully updates Bob’s phonebook with it.

Unless Bob’s phone is smart enough not to trust Facebook and adds the new number as a secondary rather than overwriting the old one with it, Bob has now lost Alice’s actual number.

Does anyone know what happens in this case?