If your site has high impressions (100k and up a day) it's nearly impossible for one single CPM ad network to fill all your inventory. So put at least 3/4 (more if possible) in a "chain" -- i.e. your major network first, it will default to your second-tier network second, etc. So that whenever your first network runs out of ads for you, the second network in your "chain" will step in, and if that one runs out, your third, etc. For instance, one of my sites does an average of 125k/day, and despite the fact that I already have 4 ad networks linked, I'm STILL getting defaults. This means I need to get at least one more, maybe two more, networks to join the chain. For the record, my first network is TF, followed by FC, then Casale, etc. Despite being a giant network, neither TF or FC combined can fill the 250k impressions/day I give them. So putting a network chain together is VERY important so you don't lose money.