I think the internet has proven time and time again that quality content is what drives users to a website. No amount of PR can make a terrible site viable. For example, Nature Network has a huge amount of PR behind it, but the activity on the site is pretty low (~3 new blog posts a day and 10 comments on those blogs). They're obviously doing something wrong if they've been around for a year, have a huge name and a large PR budget, and still don't have a fan base.

For another example, take a look at Friendster and Facebook. Frienster, at the time, had a gigantic network, better content, and more street cred than facebook, but friendster was stuck in the 1990s using slow cgi/perl causing massive network problems and everyone migrated over to facebook. People choose content ...

Additionally, I think that the population of possible users is pretty large for a niche social network. My estimates come to about 500,000 users. I say give this sector another year or two to develop and a few top players will come out. What's going to make any of these networks stick out is listening to user feedback and developing the site based on that feedback. That's how I'm trying to structure my site. I laid a basic groundwork of things that I would like to have in a network and now I'm trying on my tiny graduate student ad budget to get user to the site to help me build it into something they'd like to use everyday. Unfortunately, I don't have the constraints of proprietary software, free APIs, an angel network, or a fat cat journal CEO weighing down my ability to expand my site in whatever way my community of users decides, I just need to get those users ;)