Is this site for local businesses in your area or businesses nationwide?
Definitely nationwide and later worldwide because this is internet based business.
I like your site a lot. It is clean and very user friendly. To promote it I would go into the small business forums like this one and get the word out. You can use Meetup.com to reach small business meetup groups nationwide. I noticed you didn't a have a blog. Having a blog would be a great way to get search engine traffic locally and nationwide. Google loves blogs and this is a known fact. Then the other thing you can do is YouTube videos. However iinstead of making videos about your site, offer free advice that will provide value to those small business you are targeting. Then offer them the link to your site and explain the benefits of becoming part of your small business community website. Was this kind of along the lines of what you were looking for? Hope it helps.
I think Samuel above has made several good points - Blogs and YouTube can be very helpful.
One more thing I think would be helpful is that your site has a lot going on when you first arrive. I had to look at your "about" page, which in a few sentences summarizes your value to small businesses. I did not get that from the home page.
My other thought is - how can you get your users to promote your site for you?
If you are giving them value - are you letting them share posts that people make on Facebook, Twitter, Digg and LinkedIn? I'm not a member, but I looked at a post and don't see this option. It is a simple way to let those who appreciate your content get the word out for you.
Do you have an email newsletter? You should. Make sure it has lots of value and the subject has a reason for people to open it.
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