Reason 5,395 not to use Facebook:

[…] At first I was baffled – I guessed maybe Facebook had copied something across from my previous account via a cookie or similar. But it turns out that FB used my mobile number (which they took as a security check) to match up with people who have me in their mobile phone book and have synced the Facebook app.

I fully understand why they’re doing this – it connects new users into existing networks, it’s an evolution of the ‘import your Hotmail contacts’ facility. I just didn’t like the approach at all. They demanded my mobile number under the pretence of a security check, but then used to it find people who have me in their mobile phone contacts.

This has badly violated the privacy of a friend who has, I now know, been operating a second Facebook account to hide the fact that he is gay – something which he has good and important reasons to hide. It also showed me that an ex-girlfriend of long ago still has my number saved – which is understandable because I am great.

Facebook privacy fuckup reveals who has your number in their phonebook – alexmuir.com