You need to work big to small here, and your focus seems almost backward.
The things you mention as concerns are relatively data-light: Avoiding checking facebook through the app, or turning off image loading in the browser aren't really going to save you much unless you're hitting very image-heavy pages often*. You can spend a ton of time working to minimize these and in the end you won't save much--as a hint, if you were doing it on a 56k modem (even if it was "bandwidth-heavy" then), then it's probably not a significant bandwidth user in this day and age.
Meanwhile the things you're dismissive of are exactly the sort of things that can suck bandwidth: Google Drive can be using arbitrarily huge amounts of data depending on how you use it. Instagram is the definition of "very image-heavy", unless you're pretty selective about its use.
*Blocking videos in the browser can be a big win, but IME autoplay videos are extremely rare on mobile