I apologize for the confusing experience regarding your data allowance usage. Are you still being deducted from your GoSurf data allowance while browsing from the official FB app?
Also please make sure there are no other non-FB apps running in the background that could be causing the deduction, since with Android the background app refresh feature is enabled by default.
Your App will consume Mobile Data whether you like it or not.
Is the consumed Mobile Data of your app charged to your GoSurf 499 subscription?
Thats why I dont trust those APP ADD ON services from any telcos. I just get pure volume/GB allowance promo.