Data disappearing!

Dear Mtn, where is my data going? Topped up a monthly 100mb on 07/04/2015 (extra 55mb free for 3 days). Again 12/04/2015 top up 100mb (extra 55mb free for 3 days). 14/04/2015 my 200mb on contract kicked in, lasted about a week maybe. 21/04/2015 topped up a 500mb (extra 275mb free for 5 days), lasted about a week. 29/04/2015 topped up 100mb (extra 55mb free for 3 days). My facebook usage was up briefly during the week I had the 500mb. Where is all my data going? Do we now live in a country where smart phones have no place and can only be used in Wifi zones and keep phone data off unless there's an emergency?