AIRTIME MISSING

I HAVE A S4 MINUTES CONTRACT THAT I PAY R259 A MONTH FOR 50 MINS 50 DATA AND 50 SMS . I'VE HAD THIS CONTRACT SINCE OCT 2014.I AM MOST UNHAPPY WITH MTN SERVICE AS WE ARE PAYING CUSTOMERS. NORMALLY IF MY DATA IS FINISHED I PURCHASE MORE, AND IF I DON'T HAVE DATA I USE WI FI ,WHICH WORKS WELL AND DOESN'T EFFECT MY AIRTIME. SINCE THIS MORNING I NOTICED ALL MY AIRTIME HAS BEEN USED UP DUE TO ME USING BBM AND FB , HOWEVER I STILL HAVE 393.70DATA.<br> CAN SOMEONE EXPLAIN TO ME HOW DID MY CONTRACT SUDDENLY CHANGE.I AM MOST UNHAPPY WITH MTN SERVICE , I ALSO HAVE A VODACOM PHONE OF WHICH I'VE NEVER EXPERIENCED PROBLEMS. MTN HAD BETTER SORT MY COMPLAINT OR ELSE I WILL TAKE THIS UP WITH THE CUSTOMER COUNCIL.