Usage

There have been many Windows users experiencing repeated failure while downloading and installing that update - failure causes it to be repeatedly downloaded until success. It's but one source of "phantom" usage and there are many others. The update is simply one of the bigger ones.

When you installed successfully, did you go back and check to see if metered WIFI connection was still set to on if applicable? Apparently the anniversary update resets it to the default (off) since the setting is now applicable to individual WIFI connections.

As far as only the modem being plugged in, check the usage periodically usig an alternate connection (not Exede) to determine if usage is actually accumulating or not while only the modem is plugged in (you do have to wait for the last bit of usage to be reported when simply unplugging a router from the modem - after that no usage should accumulate with only the modem powered on).

There are numerous threads here that describe how to determine the accuracy of the Exede usage meter - some of them are just to right in in the sidebar here. 
  

Actually, it took three of four downloads of the updates to finally take on this computer and on my desktop the big update still haven't taken. I restart it every once and a while when I think about it. Luckily none of this is on Exede as they've went through a lot of gigabytes.

James, we are not  talking about the Windows 10 system upgrade back last year through July of this year, but the Windows 10 Anniversary upgrade, which has  been downloading over the last two months, and will continue through November. It is the major upgrade to Windows 10, and as david has stated, it sometimes fails and then downloads again and again until it finally installs successfully. It is a big data killer for those of us on metered service.

Also beware of Microsoft's new cumulative update strategy for Windows:

https://community.myhughesnet.com/hughesnet/topics/windows-users-face-update-bloat-and-tough-choices

Applies equally to Exede.

Thank you for that link, Old Labs. Microsoft's new updating strategy will most certainly cause many more people to be alarmed on where their data usage is going. To make it worse, many users have commented on the fact that the latest Anniversary update has taken 4 or 5 times before it actually installs completely, meaning it downloads the same data multiple times. Way to go, Microsoft, there are a couple  million of your customers who are on limited data ISPs.

See Steve's first reply to the original poster - doing it yourself or having a moderator do it for you will yield a generic, categorized summary of your usage which might yield some insight (however often not detailed enough to take specific actions to eliminate what is often unwanted, unnecessary usage).

That requires digging a bit deeper and knowing something about your environment, and you won't get much assistance from ViaSat for issues that fall out of their responsibility area. 

Do you use a router and if so what brand and model?

Which operating system (or operating systems if more than one) do you utilize? For example Windows 10, Windows 7, Window 8, etc.

Do you have a second line of defense for your primary anti virus application? 

In addition which browser(s) do you use?

You may simply need to ensure those aren't using data in the background and that you may simply be unaware of. Many times it's unnecessary data or unwanted that can be tamed easily.

And which Exede Plan do you have?

But right now, your mention of Facebook is raising a red flag for me anyway and setting your Facebook account to not autoplay will yield some usage relief - there are other Facebook possibilities but require lnowing something about your specific  environment (especially browser).