There is no singular "it" for the Apple Macintosh computer line.
Like other computer related equipment Apple offers a range of products starting with a entry level and ending with their most expensive top end model.
As to Facebook using a lot of data ... well it CAN, depending on settings and user habit.
Auto-start of videos can use a lot of data. That function can be turned off as well as ad blocking extensions for browsers will reduce unnecessary data usage.
There are programs such as Facebook Purity that can be installed that allow easier access and finer control of Facebooks "offerings".
Is your router password protected ?
Besides the above do you have encryption enabled on the wireless portion of your router so that only authorized devices can connect ?
Carefully go through all settings on all computers and devices to ensure that all cloud and sync functions are disabled.
When shutting down do a true "power off" not just sleep or hibernate.
An average of 333 MB per day is going to take very careful management.
Having phones with access to your Wi-Fi can be a big problem.