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I'm afraid it does big style
There are a lot of very successful PFI directory based niche portals/vortals/directories though most of the successful ones, don't just offer PFI subscription they also offer pretty much a full array of web services. Hosting, Affiliate scheme's, Design and SEO (brouchure to data driven sites), on spec apps, etc.
Some in my industry include:
Thomas Regional (there is a member who works in the NY office). Industrial based.
1st Directory. Industrial
Applegate (UK based). Industrial
Category, or more correctly Industry is very important, as these are successful players in my industry, though i don't think that much of there SEO technique's, there scale is massive, usually with multiple websites, pushing slighly different niche's.
But then you could look at.
Law, Insurance, Sex, Computers, Science, Publishing, Information (markets), etc
They would have similar backbone considerations, but there would also be industry specific techniques for there marketing and considerations towards potential customers.