Pre-installed apps cannot be moved to SD card. The app itself lives in a different partition (/system), which is why you can't move them and also why the app itself isn't using any of your space. Its data do (but those don't move when you move the app), and updates do it to. Disabling a system app will remove its data and updates, so that does save you space.
In fact whether user-installed apps can be moved to SD is up to the developer: not all developers allow this, and "moving" those that can be moved actually only moves part of the app. So I don't know whether the system would let you disable Facebook and then install another copy and move that, or whether, if you could, it would save you any space. It might be that disabling Facebook and installing Facebook Lite (or other FB viewer) would save some space - I've never tried, so can't be certain.
Doing a bit of research, based on what you've posted, I've found an X-BO V6 (Android 4.4.2) and an X-BO O6 (Android 6.0 according to most sites, 5.1 on some). The V6 sounds more likely, based on the OS version, but both are described as 4GB phones rather than 8GB. You don't happen to have a very small (4GB) SD card installed do you? Or is this a different model?
I don't know where "Android 7" came from though. Neither is advertised as such. Did it say that explicitly on the box, or did it say something like "A7" (which would be the cpu core architecture)?