Even if it doesn't have it from these sources, Facebook friends who have given the app access to their own contacts book may also have given it your number. Given that most of us have hundreds of friends on Facebook, the chances of this happening are pretty reasonable.

As Facebook says in its online Help Centre:

When we ask you to add your phone number, you may see your number automatically suggested so that it's easier for you to add. This might be based on your phone or tablet, your mobile operator, contact information provided by others on Facebook, or other sources. We also might have your number because you gave it to us in the past, but haven't confirmed it. The number will only be added to your account if you choose to confirm and verify it.

Giving my name away

Facebook says it only adds a phone number to an account if users confirm it, but it turns out that even unconfirmed numbers can be used to identify people. After finding Facebook knew my number, I discovered that a colleague could easily find me on the network via my phone number, even though I hadn't verified it.

Here's how. When a Facebook user tries to add new friends on their mobile app, one way to do so is by importing numbers saved on their phone. Facebook matches these numbers to profiles, and suggests adding those profiles, displaying the user's picture and full name.