It sounds like the problem you are having is user retention. You get people to your site, but they don't end up sticking around to use your tool/service on a daily basis. This means that you do not have a strong enough value proposition (or you are marketing to the wrong people, but more likely the former).

Try to get feedback from the people who tried your website but then left. Repost to the blogs you posted to and try to see what the people who tried it think, and why they didn't up leaving.

Also, make sure your website is solving a real problem. Your site may be 'useful' but unless it solves a problem that currently really really frustrates people, nobody is going to want to spend the effort to go to your site and use your tool unless the problem is solves is really big, and your solution is awesome.

I could give more specific advice if I knew anything about your site. Good luck!