Who is your target audience? If you don't know who they are, then your startup already has a problem... Once identified, can you think of any bloggers or other people that audience is inclined to listen to? If so, why not try contacting those individuals personally and ask them to take a look at your site and tell you what they think. Sending out a form email in this sort of outreach would be plain stupid and your response from them would be near nothing as you would deserve.

If you take the time to study these folks, you'll do much better. And if you're unable to convince them that your site's worth their while to use, then it probably isn't going to be a hit with your target group. Compared to any of the mass media approaches, this will cost less and work better.