I love the big sites like MySpace & Facebook for their incredible and innovative impact on the web and social marketing, I also hate them for being such a mess to get anything really useful out of. Niche social media sites provide the missing link in social media. . They are the intermediary between Googles high relevance & Diggs “whatever goes”.
Briefly, I think that as long as the large sites like facebook and myspace allow the capability to create private sub-communities such as “companyname”, “familyname” where you can break off from the “world” and engage with your deeper “interest” I am confident that the big sites will maintain their appeal. However, it needs to appear as if the sub-community site is “standalone” when you enter that social zone. my two cents.