If you ask me...the simplest solution would be to use ning.com to help establish your own social network site in a niche.

I'm not trying to advertise for them, but I have used them. I even pay an optional $4.95/month just so they can route MY network through a pre-existing domain name I purchased. That way, when people visit my social network, they don't have the 'blahblah.ning.com' in the name.

Now, as far as creating a network in a niche that you have a presence in...

If you already have a presence in the niche, you need to start plugging your new network on the back end of all of your websites and/or blogs that you have in that particular niche. Start capitalizing on the targetted traffic that your sites are already getting, and leaving a link or a banner, or a text ad for your new network on member pages, blog posts, even in online forum signatures if you're an avid forum poster.

If you have a sales website...put an advertisement for your new network on the 'thank you' pages you have. That way, you'll know that EVERYONE who sees the ad will have done something beforehand that shows great interest in your niche, regardless of whether they're new to you, or a returning visitor/subscriber/customer.

Also, for some extra assistance, try working out ad swaps with other site or ezine owners in your niche. You promote their biz on your site (or to your subscribers) and in return, they tell their constituents about your network.

Who knows? Your ad swapping partner might be a good addition to the social network that you're trying to create. Make him a partner in that, if anything.

Give him/her an ad spot on your network, in exchange for an email to the subscribers or something similar to that.