Well, I have noticed a trend among Winblows users that they use P2P to download a LOT of pirated software and applications (so called warez) and a huge amount of movies (read pr0n). If you visit all major Indian torrent forums, you will see that almost all of them have some type of broadband connection. As for me, I do mail (which eats up a lot of traffic), then IRC (which is also pretty heavy considering the number of channels and servers I am in) and APT (which is also considerable). That eats up 1 GB each month, many times even more than that when I dist-upgrade or something.
Well I have been using Linux since the days I did not have an Internet connection..and believe me without an internet connection its pretty difficult to get a Linux box do all the things that a windoze box would do(I am talkin about multimedia capabilities..for office stuff its fine coz OO.o is mostly preinstalled). Also the various dependencies etc take up a lot of downlaod MB's. I have a friend of mine who was actually finding it very difficult to manage all his yum downloads within the 400MB limit in TriBand. I am more fortunate to have unlimited download internet connection. What I wanna say is that average bandwidth usage is actually more in Linux than Windoze(p2p excluded) as the most common software in windoze like Winamp will be readily available from a friend as compared to say xine-libs ,libdvdcss and kaffeine!! (but what the heck..I still like Linux )For my part ...I need around 4-5GB per month for my Linux updates plus music downloads.