Vodacom ***** my money for data but i do have data.

Everymonth i see vodacom send me a message of 67c for out of data use today almost R20 for their out of data use.I do have prepaid and if there is no data then i cannot go onto fb or whatsup.This is becomming more and more and it looks like vodacom is *****ing from their customers doe they want me to stop using them and go to mtn or cell c.They must explain to me for what that is and how it is possible to use data when it is finnish.