Several estimates floating around (*) suggest between 60 and 100 PB (petabytes) per
month of US backbone traffic, which works out to 180 and 300 Gb/s average traffic.
Andrew Odlyzko
(*) See my papers at <http://www.dtc.umn.edu/~odlyzko/doc/networks.html>, or a recent
(and about to be updated) report from RHK.