I want to know how Facebook discovers the people who you know in real life or who know you. 

I tried the following to see if Facebook can still discover my acquaintances in real life and suggest them to me as a friend.


I connected using a VPN (an anonymous VPN, not one of those free VPN services). I have confirmed that it does not leak my actual IP address.
I cleared the cookies in my browser (specific to sites like Facebook, Google, and Yahoo) and started a fresh instance of Browsing Session. Anyway, cookies specific to Facebook only should matter, and I cleared them all.
I registered an email account with an email service provider who does not require a mobile number for registration. I used an email ID name which had no resemblance to my real name. I did not mention anything related to my geographical location while registering the email address. Please note that this was a fresh email address, and I have never used it to send an email or receive an email.
Now, I registered on Facebook, using a name which does not resemble my real name in any way.


However, Facebook requires phone number verification before you complete registration on Facebook. This is the only place where I specified my real phone number to receive their security code.

Once, I completed the verification. The moment I logged in, I could see Facebook giving me a list of suggestions of people I may know. It was surprising indeed, since this list was extremely accurate. It included people I knew in the past as well my current acquaintances. It makes me rather suspicious.

The only way I see that Facebook was able to identify the people who may know me or I may know them was using my phone number. So my assumption is:


They appear to have a deal with the telecommunication providers in different countries. Once you disclose your phone number, it looks like they get access to the entire list of phone numbers with whom you have corresponded in the past. Then, they do a second level lookup to identify the Facebook profiles of those corresponding phone numbers.


Also, interestingly, there are some people with whom I may not have ever corresponded with on phone. But of course, Facebook can find them through other people I know and suggest them to me.

Am I correct that Facebook was able to do all the correlation of people I know in real life using my phone number?

It would be interesting to see whether they could still correlate it if I use another phone number.


  So, yes, they appear to have a deal with the Telecommunication
  Providers in different Countries.


Well that's ONE explanation.

Another one that I like better is simply that they have all their users' contact lists, thanks to their mobile application which no doubt reads everything and sends it back to their headquarters.

All they have to do after you register with your real phone number is look through all those contact lists, and find the people who possess your number.

This idea that they may have arrangements with telecom providers seems a little far-fetched to me, in great part because it is simply illegal in many countries to disclose phone records to anyone without a court order.
