1. Pay people to create the initial content.

2. Create it yourself using multiple accounts.

3. Have a following like the StackOverflow guy did.

I've had success with growing it one user at a time, starting with yourself, friends and family. If your startup provides something of value, it will grow.

If it's not growing at all, even a couple users a day, then that hints towards a problem with your product/service/site and you need to get some feedback from your users. Some of your assumptions when creating/designing your site may be off.