Why does my iPhone go through so much data in a few days?

I have an iPhone 5, which is running iOS 9.1, and I seem to be going through 1gb of data in just less than a week. The only Internet browsing I do on the phone is a bit of Facebook each day (I also upload pictures to Facebook from time to time), and a few Google searches each day.

Could it be the Facebook app that is eating so much data? Or could it be something else?

In Settings, you can see just what is you data hog, and shut those off. And whenever you can, use Wi-Fi, not your own data - shut it off. Do not let any apps automatically update, unless they are CRITICAL - I run my AMEX update, because my card number has been jacked before. All your other apps you can update in seconds with a swipe. Did you BOTHER to read the book that is on your phone, about how to use it?

When I'm at home, I shut off data and strictly use Wi-Fi. When I'm out of my home, I only turn data on if I absolute need it. If I need to use an app, I turn it on, when I'm done with the app, I turn the data off again.

Try disabling Wi-Fi Assist

Also should take the time to check & change which apps you allow to use your data connection.