If it's not a commercial secret, I wonder, are sites like Reddit and Digg low margin businesses, or are hardware/bandwidth costs a tiny speck compared to the ad

If it's not a commercial secret, I wonder, are sites like Reddit and Digg low margin businesses, or are hardware/bandwidth costs a tiny speck compared to the ad revenue generated? In your "IAMA" thread on reddit, someone estimated that Reddit must be pulling on the order of $1M per month, making the costs neglibile. Digg must be earning even more, because of its greater number of users and their higher susceptibility to ads.