Today was quite a milestone because we just went over 10 million RTB API requests and 500,000 add-on downloads. (I say it happened today, it happened a few days ago but I forgot to check). An API request is anything that your game has to ask RTB about - this means client auth/server auth/mod manager browsing. 10 million is a lot and at peak times we get about ~10 every second.
Since 2009-09-20 (our stats only go back that far for some reason) we've also used 157GB of bandwidth, which doesn't seem like a whole lot but when it consists of ~20kb add-ons and screenshots it does add up. On average we're using ~2GB per day, 4GB on weekends.