This might be the answer to my increased data usage. I have never, ever used Siri, and always had it turned off. However, per your post, I just checked Settings and Siri was turned ON. If somehow the iOS8 install turned it on and it's using data in the background that I'm unaware of, then that would explain things. Anyway, I turned it OFF and will see what happens next billing period.
There is a way in iOS 8.0.2 just tap Settings - Cellular and scroll down to the list of apps. Under each app it shows how much data it's used since the last time you reset the statistics. If you want to see how much data the system services use, just tap it and likewise it shows that on the next screen.
On my iPhone 5c my biggest data users since my last reset on Oct 13, 2014 are:
Chrome 53.8 MB
Facebook 31.8 MB
Argus 15.7 MB
System Services 16.1 MB (this includes all system services)
Siri 596 KB
The thing is I don't use the FaceBook app that often, yet it's one of the biggest consumers of my cellular data. Now there are toggles next to each app that are supposed to restrict that app from using your cellular data and only use WiFi. I haven't tried that yet so I don't know how well that works.
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