Not to mention is that loss of users impacts another important revenue source for many sites other than just ad networks and that is sponsored content, referrals and syndication. If I'm running a full out adblock and can't access your content I can't read sponsored content, and I can't share your content with others that might not be running adblock, or do and will be more inclined to disable it, or to even pay for that content through other means.
Boycotting rarely works, and boycotting your customers is pretty much the dumbest move any business can make.
it's the same reason twitter started turning off ads for influential people.
1. browsing hacker news sees interesting article, uses ad block.
2. posts link to Facebook
3. link gets propagated across Facebook and clicked by 1000's who do not use ad block
4. x1000's
You can think of ad block users like carriers. They show no symptoms, but could be the man vector for distribution of your content. So they may have a value that can't be measured in ad clicks.
The interesting question is if they saw a subsequent fall in ad revenue indicating a correlation between ad block users and more ad revenue.