1-Log in to your face book account.

2-write the mobile number you want to trace in search bar and click on find.

3-If the person whose number you want to trace have registered on facebook using his mobule number then you will surely find him.

I also find a location of a Telenor number using this trick.
Note: you can find mobile numbers of all network operators by this method(warid,ufone,jazz,zong,telenor,docomo,airtel etc.)