I can't for the life of me understand how they would do this. They have employees which means they need to make money, so to suggest they are paying the publisher 100% revenue share makes it difficult to believe.
I have noticed that even though I serve around 6,000 page views a day with cpm asking rate of $3, I am not making anything close to what I should be earning, even if we assume, say, 20% fill rate. My ad is right at the top of the site (so its above the fold) and is getting exposure to every visitor that comes to the site. I can only explain the discrepancy with an explanation that they must, in reality, be taking a significant share of the revenue. I don't mind if they take 50% even, but they should be open about this rather than say the publisher is getting 100%.
And at $3 cpm on a quality health site that attracts lots of genuine comments from visitors, I really don't see how they cannot provide a 100% fill rate with the ad showing in a premium spot. In theory I should be earning $18 a day at $3 cpm; and lets assume Contextweb takes even 50%, I should still end up with around $9 a day from them for that ad spot. However, I am not making even close to that.
However, they are easy to get into and they provide backup for adsense and other networks like Chitika for impressions they cannot fill themselves.