Social networks have the ability to hyper target people based on keywords, locations, and hashtags. One of the best networks to drive traffic to your site would be twitter. You can see this video I did on 3 tools in particular that is allowing us to get 50 registrations to our own online community: https://www.youtube.com/watch?v=jBGypRKBUD8

Additionally, LinkedIn is really good for their search features. You will be able to search based on your criteria, and connect with people. The, through private message campaigns, you will send out messages to these connections introducing them into your concept.

I also recently found out how to send direct messages to members of LinkedIn Groups, and this could be another source of registrations to your site.

Hope this helps!