They say that there is someone for everyone. If that is the case, some of the more mainstream sites may not cater to one's peculiarities. For example, there is FarmersOnly.com, a site whose tagline ("City folks just don't get it.") says it all.
SuccessfulMatch.com has its own network of niche dating sites, including Deafs.com, SeniorMatch.com, BikerKiss.com, EquestrianCupid.com, and the mother of all sites: PositiveMatch.com. That last one is for folks with incurable sexually transmitted diseases, such as herpes, HIV/AIDS, and HPV.
When users sign up for the site, assurances of anonymity are provided. However, in the site's terms, it says that it can share profiles and user data within the company's network, which includes a number of "affiliate" sites, each of which are created by third-parties for new niches.