This is the essence. There isn't a single marketing guru that can make you're site popular unless it is worth visiting.
Rule #1: Content is the King.
Then you can focus on both paid and free advertising models. Start spreading a word of mouth on forums, blogs, social networks (guerrilla marketing), or pay someone for decent viral marketing campaign.
You can pay for PPC campaigns and/or do a decent
SEO for good placements in organic results.
You can motivate your current community to spread the word around and fetch new members. Offer reputation, value-added services or something else of importance to them.
But most importantly, think of a reason why would
you register on such a website. Any particular reason? That is also what everyone else thinks - if you answer that question, you did 80% of the job.
Bookmarks