Are all them "middle men" so to speak connecting publishers to other ad networks like valueclick, tribalfusion, etc? or are they there own affiliate program connecting advertisers to publishers? if it's the former, wouldn't it be better for a publisher to apply directly to the ad networks? it seems these 3 companies are used by the top web sites. thanks.
I believe a few of the smaller ad networks will get their own advertisers plus they will also use 1st tier networks so that they can full their network with more advertisers. That's how many networks first start up I think is by using 1st tier ads so that it looks as if they are fully populated straight away. The more advertisers they get the better and the less 1st tier ads they are likely to use. That's why I prefer to join tier 1 networks, but it's not always that easier as some publishers find it harder to get into the 1 tier networks than the smaller networks.