I) Small Business - Businesses are starting to recognize the value of using social media in marketing. Facebook and Twitter especially are 'musts' for most small businesses. While these 'shotgun' approaches are useful, getting involved in niche networks/communities provides a much more targeted environment where it is easier to establish your brand and connect with potential customers.
II) For regular social network users, there is value in having a community and platform that is specialized for a particular purpose. If I like reading books, I may like to join a community where my profile is a bookshelf and I share reading lists and favorite authors. If I use a traditional search engine to find one, I come across mainly dead links, sign up pages, and other clutter making it hard to easily find the best one for my needs.
Rawsyntax - The second point you make has been a major concern in initial feedback. One issue is that Amzini actually has to create awareness that there are social networks for almost any purpose before its value becomes apparent.
Initial monitoring and feedback suggests Amzini has been able to pique interest and acquire new visitors, but metrics for repeat visits have shown signs of problems.
Now that I've explained the value/target audience further, do you see a need for such a service? And if so, how could the site be changed to better convey the value to you from the onset?