I started a shared data pool plan for my family and my brother's usage was estimated at about 2GB per month. A couple weeks into the billing cycle I checked usage and my brother had used MORE data than the other FIVE of us combined, and was on track to use over 5GB! We talked about it and it turned out he had the new facebook app installed and complained that the videos had started autoplaying. He found it annoying. We did a quick search and found that the DEFAULT setting is to autoplay videos as you scroll past them, regardless of the connection type.
We changed the setting to "Wi-Fi Only" (or never) and nothing else about his usage. His average daily bandwidh went from 150MB to 50MB.
Facebook's new, annoying, default setting was on track to add 3GB PER MONTH of data usage! (30 days * 100MB)
We were lucky to be on a new plan with 6 people that I was monitoring to make sure we had the right data plan. An extra 3GB of data sent to a casual users ought to earn Facebook some kickbacks from cellphone providers!