After a presentation about shortly after the book was published, an audience member came up to me and said that we didn't include a mention about a specific portion of the Invisible Web.
I thought to myself and wondered what we forgot to mention.
She explained that for many searchers EVEN IF the material has been found and crawled by a general web spider but it's not found in the first few results, its invisible.
I think the comment was right on the money. Placing content in a database is one thing, the ability to FIND it is something else. The fact that:
+ the average query length is about 2.8,
+ most searchers don't use any advanced syntax
+ search skills haven't improved that much in the past few years
+ there is little to no use of controlled vocabulary (to help bring like things together)
+ many searchers only look at the first few results on a serp
and other issues further complicates the situation.
This is another reason why specialized/focused search tools (verticals) can be very useful.