I have gone over my cap for the first time and the daily increase started a day or so after upgrading my iMac to Mavericks. I also upgraded my mother's Mac mini at the same time and she has hit 80% and again the daily data both up and down is way more than she ever has used. She often doesn't use her computer for days on end and then when she does it is usually only for email.
I am out of the country and have texts from home about internet slowing to a crawl. Won't be able to get to the computers until getting back to NZ on weekend.