Clickety Clack comments on a ZDNet post that there might be a roll-up opportunity among smaller advertising networks. Implicit in his post is the concept that rolling together a bunch of 30-40% reach vehicles might give you some sort of 70% reach vehicle. That might happen, and I am certainly painting in fairly broad strokes here, but a lot of the inventory on these sites is Right Media and MySpace. To my way of thinking, almost anyone can rapidly build a network with 30%-ish reach by doing low frequency buys on MySpace and Right Media.
The real question is what the overlap in their advertising base is. If they actually have differing demand and are not all working the same CPA deals, there could be some leverage to a roll-up there. Unfortunately, there is basically no way of knowing from the outside what that opportunity may be.