I don't have much experience about SEO or such advertising, but I hope these tips which I learned when building my QA site could help you:
- You should ask and answer by yourself using guest accounts or your clone account.
- You shouldn't copy/grab entire contents from any QA website such as StackOverflow, Yahoo Answers,... Instead, you should create question with same title but different answers or some "copy-writing" tricks.
- Invite your friends to join (and help you building too!).
- Improve your website's SEO by building great content, backlinks or any ways you know.
- And as Donox Btz Gvr mentioned, you can also using online advertisement such as Facebook Ads.
- And be slow, it may takes 1 month or more before you can reach your user.
EDIT: - For building contents, bulk data imports,... you could use BCG Bulk Content Generator plugins.
Good luck to you!