I had my mobile phone-number appear pre-filled in an add-your-number-to-your-account prompt on Facebook's mobile website while I never provided it in any way to Facebook myself (in the meaning of: neither added it to my account nor mentioned it ever on the website; I never used their apps at all either) . They had farmed it from one of my contacts adressbook obviously. Not surprising that they'd do that but still a disconcerting feeling to actually see it happen.