Before you launch a job board (or any site) I think its important to lay some groundwork before you go live. Specifically there are two important marketing tactics you should employ.
A blog and an email list.
Before I launched CareerCloud.com in January of this year I began by setting the domain to Launchrock so I could funnel people who were interested in what I was building to enter their email address. I did this beginning in June of 2012, a full 6 months before it launched. This way I could tell anyone I met pre-launch to just hit the site and signup.
By the time January rolled around I had gathered 2000 email signups! I was ready and armed for the coming marketing fight. Launchrock is a free service and its awesome at collecting emails before you roll out your site. Use it.
Beginning in September 2012 I also started a blog (blog.careercloud.com). The blog allowed to write about social recruiting and became content that I could immediately start sharing on social media. This drove traffic to the site and hence more signups. Plus the site had an established domain with actual content when we went live. It was great for SEO.
So do yourself a favor and follow my lead before you launch. Leverage your social media connections and spread the word about what you are building BEFORE you go live.