Free FB is FREE but there's a bait especially when you don't know how to restrict your data/cellular access.
Once data/cellular is turned on, ALL applications which are capable of connecting to the internet will use your data/cellular connection. Once those applications will successfully connect to the internet, sometimes once your data/cellular is turned on, your available load balance will be used/consumed immediately. It happens for both prepaid and postpaid unless registered to surf promo.
Kahit naka restricted ang cellular connection, mababawasan at mababawasan pa rin ang load mo so, kung prepaid ka, wag ka na magload if you want to use free FB.
Since postpaid ka po, just restrict the apps for data access. Note that, you will still be charged despite being restricted.