Another day, another vertical. Earlier this year the press release mill worked overtime announcing that every imaginable media brand and ad entrepreneur was launching some sort of vertical network—either of content, ads, or (usually) both. Martha Stewart has her "Circle" of blogs and small sites. Forbes claims it will gather more than 400 financial analysis sites under the same ad sales umbrella. IDG has TechNetwork. John Batelle’s Federated Media slices scores of premium blogs into a host of channels. NBC even created a vertical network around its TODAY Show. Stay tuned for the Matt Lauer Ad Network.

The temptation to go vertical is fueled in part by turnkey services such as SyndiGO and Adify, which let any publisher come in and roll their own ad network out of a series of partners. The barrier to entry is extremely low at this point, and so everyone rushes in. But is this vertical fad a good thing, or another in a long series of internet gold rushes?

The theory goes that traffic patterns online are shifting eyeballs away from major destinations, and so the business models must adjust. Audiences fragment across ever more blogs and small sites as the authority of trusted brands diminish. Search engines radically changed the way people find information, and they lead us way off the big brand reservation. To recapture that dispersed audience, both big media and ad buyers have to broaden their reach and find ways to systematically re-aggregate these eyeballs. Publishers can acquire or partner with relevant smaller sites in order to maintain their reach.

The vertical ad network model can work well for small content providers such as bloggers because it gives them the imprimatur of a major brand and promises better revenue than the Google AdWords they already have. The very hot Glam Media network, for instance, sells big name advertising into literally hundreds of one-person celebrity gossip and fashion commentary sites that a media buyer would ignore otherwise. Netshelter, Inc. aggregates nanotargeted sites such as IT Community Bytes and 2CPU (dual processor news), but can sell into them major rich media placements from Microsoft. Netshelter claims a reach of 8 million uniques, which rivals CNET Networks, Inc.

For an advertiser who hasn’t the time to seek out the ad reps at all of these niche properties (if there is one), the well-run vertical net seems to serve them well. Most of the agencies and media buyers I ask rely on one or two such networks in a given category such as auto, health, or travel to reach these audiences. The networks themselves claim that visitors to a highly specialized site in the long tail of content are more responsive to advertising because they are more engaged in the subject.

So what is not to like about this vertical ad craze? First, that it is a bit of a craze. There just are too many of them, for starters. How many ways can you slice the audience, after all? How many financial services, IT, or women’s content verticals really can achieve the necessary scale? We already have seen some poaching of sites, where one network brags of having grabbed this or that blog from another. When you have hundreds of sites under your brand, who is vetting them for quality? Is an eyeball on a blog really worth the same as one on a branded media site?

The small site owners have to stay on guard. A major network or media brand may take over ad sales at your site, but will they give you a fair shake? Whose inventory gets sold first or hardest by that sales team: the $20 CPM banners at the branded site or the $5 CPM space on yours? And is there any loyalty to the smaller guys? Will a network drop a site as soon as it loses favor (or Google hits), or will it partner with this self-starter to improve quality and ad-appeal?

To be sure, the vertical ad network is nothing new, and there is ample evidence it can work for all links of the value chain. Long-standing companies such as Jumpstart Automotive Media, and Travel Ad Network get high marks from advertisers, and they spend time servicing their own publishers. But they also maintain a controlled pool of publishing partners they can service and improve. Ultimately, throwing ad technology across thousands of blogs is not the answer to fragmentation. Strong content is.