My wife and I have had our iPhones for nearly 2 years. I have an X and she has an 8+. We have consistently used +/- 11 GB of data over that time period. Our usage patterns have not changed. Beginning in June (so the last 2 months), we have begun using +/- 16 GB. The Verizon app's Data Hub shows that it is mostly coming from social media apps and pretty evenly between my wife and me. Facebook is the only social media app that I use. Has the FB app started using significantly more data? I upped my plan from the 12GB to 16GB plan last month and still had to dip into carry-over data. Not cool.
How you are using the phone might not have changed, but an application update could have changed how that application used data.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
We can relate to your need to keep your data usage under your control. What tikibar1 recommended is our recommendation as well. Also, as applications update, so can their settings. Just because you aren't watching a video that appears on Facebook or any other social media account, doesn't mean it isn't consuming your data. Anything streaming in your feed, consumes data. Switching the setting to off, will change your usage.
Does this clarify things?