&gt; Facebook goes through your phone book and checks them against other users&#x27; phone numbers - you give it permission to do this when you install the app.<p>&gt; &quot;What Facebook does and what Tinder does is go through your phone book, upload your phone book and does cross-correlation between your numbers and anyone else&#x27;s number - and suggest people based on that,&quot; he explained.<p>That seems pretty clear. They grab your WHOLE address book, so there is an established link stored in their database(s) between you and every other person stored on your phone. From there, it&#x27;s just a matter of graph traversal.<p>This is one of the reasons I decided I would stick with a dumb phone and why the mobile web (or using the web from phone or tablet) is so ripe for abuse.