How you get users depends on what your goal is. It looks like your site offers a free service and has advertising (AdWords and selling ads directly [hint: telling users to name a price is not a good idea.]) as it's only income source. This pretty much rules out paying for users unless it can be done very cheaply as you aren't going to make much per new user.<p>SEO is good. A properly configured site can work wonders, but beyond that, a good product will "SEO itself". Spending time on linkbuilding campaigns or blackhat SEO will not be efficient in the long run.<p>Buying ads could be expensive and wouldn't be worth it to me. That would depend on where you can advertise and the cost, but I wouldn't do it.<p>Spam is an obvious no.<p>So what can you do? Well, this post right here will more then likely get some visitors to check out your site (if that was your intent, +1 sneaky). I'd focus on creating the best product you can, including a better design, alternative monetization methods, and asking HN or current users for suggestions on improvement and implementing them.
[sidenote: I don't have many projects I consider finished, only paused if I'm not working on them. Stay open to future development. If you aren't adapting and improving, you're falling behind.]<p>You could contribute to debate forums or other relevant websites, posting genuinely helpful comments relevant to your site and harmlessly introducing it. I personally like to only use advertising methods that I would not be annoyed with, but if your only goal is to make money, just spam your site all over the place. You might make a little cash, but you'll get a bad reputation doing it.