Ad networks cater to people buying advertising because that’s where the money is flowing and advertisers care about where their ads are being placed. This is why vertical ad networks make sense, they vet destinations for appropriateness and fit. SyndiGO is another one to watch; they build a vertical ad network for you as an advertiser.
Similarly, Travel Ad Network should also roll up a bunch of travel properties to get a better footprint, and a vertical integration. The Travel space is slightly more complicated, because of the considerable clout of the vertical search engines, of which, Kayak also intends to pull together a vertical ad network.