Focused, niche-centric social networks appeal to users. The Facebooks, Youtubes, Twitters and the like encourage big social networks and interaction and mass content production. The challenge is that mass content production forces the user to mentally filter out unwanted content. This takes time and effort and can be draining. Focused social networks on the other hand have a reduced level of content that is more relevant to what the user is looking for.
For example Athlinks.com is a social network around runners, bikers and the like. Your friends are all interested in the same thing and the content is much more likely to be on-topic to running and related athletics. When the user goes onto the site they expect to see athletic content and that’s what they receive. This saves time and mental effort and so is easier to use.
LinkedIn is another example as the site is focused to business relationships. Still this is a pretty broad subject, but its more focused then a general Facebook account.
The large social networks have been explored, launched and are successful. The next pioneers are the focused social networks. All domains are open for exploration.