In 2009 the average U.S. broadband household downloaded 7.27 GB/month, according to market research firm IDC, a figure it expects to grow to 12.5 GB/month by 2013. However, looking at the average is deceiving because the mean is undoubtedly much lower. Using a simple “80/20 rule” (20 percent of the users consume 80 percent of the traffic) results in the top 20 percent of users downloading 29.1 GB/month (growing to 50.3 by 2013) while the lower 80 percent download 1.8 GB/month (growing to 3.1 by 2013). A 90/10 split results in the top 10 percent of users downloading 65.4 GB/month (growing to 113.2 by 2013).

From: http://gigaom.com/2009/12/27/can-usage-based-broadband-billing-be-done-fairly/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+OmMalik+(GigaOM)