Segmentation of the Top 100 Sites in the US
blog.directededge.com
blog.directededge.com
Maybe there is only one Music site in the top 100, but the aggregate for Music may well be much higher. That is, what's the aggregate for each segment, what's the average and the stddev.
From this you'd see what segments are more fragmented than others -- i.e. there might only be two "Auction" sites because eBay has it sewn up. There might be only one music site, because there are 1,000s of music sites and nobody's dominated.
Furthermore, users are willing to rate, there is a lot of tagging going on, the tags are quite detailed, and the benefit from being able to recommend specific items for specific tastes that are not widely found, means that these 'niche' users are easy to monetize.
Your service is perhaps a bit vague at the moment - when you spin off a section that does targeted porn recommendations, you're going to become instant millionares.
Often when I do stuff like this I like to post it somewhere so that it gets picked up by Google and can save someone else the time later on should it be useful.