A few weeks back I commented on Facebook taking over Whatsapp. As you can see my concern was that only if Facebook knows your phone number, it can datamine on the combined databases. As long as Facebook doesn't know your Whatsapp ID, your phone number, it cannot correlate the events in both social networks.
But I was naive. Chances are that facebook knows your phone number. Just recently I had to use a Facebook account. I don't have one. A few years ago I had a Facebook account, but I had Facebook delete my account.
But I needed to have a Facebook account in order to get access to a free wifi spot, I had to logon using a Facebook account. So, I created an account. And at that moment I found out that I had to enter a phone number in order to receive an SMS authentication token. Without a phone number I could only register by sending a copy of an ID card to Facebook. No way, I was in a hurry.
All I could do was enter my own phone number, because I had no time to get a prepaid sim.
So Facebook now knows my phone number. But, I created a fake Facebook account, with lots of strange likes.
And behold: Facebook suggested I became friends with a lot of people I know in real life... What other information in my account other than my phone number related to these people. My phone number must already be there, hidden deep within Facebook's treasure chest. I can hardly imagine my friends posting my phone number on their accounts.
Facebook never deleted my phone number or my friends network. So, am I worried about that Facebook - Whatsapp deal? No. I just kill my Facebook account again. I don't want at. And if I need another Facebook account, I will make sure to have a prepaid sim to use.