It's pretty hard to get over 5GB of usage using your phone as intended. Not impossible, but not easy. I'm sure the vast majority of the top 5% of data users are tethering.
No it isn't really. As I mentioned before you can stream Netflix through the phone and hook the phone up to a TV. Watch a season or two of your favorite TV show and you will be well above 5 GB. No jailbreaking or tethering required.
This might be more difficult with Verizon since with the slower maximum speed users may be less inclined to watch video on their TV from their phone.