For example, this site ("Clarity") would be useless with only very few experts on day one.
The question is how to make sure there's a critical mass of content so that users won't find an empty page.
You can either
1) load it with other peoples content, or fake content to prime the pump.
2) recruit an initial group of users to create the content or provide the supply.
It's really that simple. For Clarity I recruited 1000+ experts before launching the site.
It's not easy, but not impossible.
I think you have to try a combination of approaches to first front load the site with content depending on the type of content you need (listings, reviews, articles or posts, comments). People don't want to see an empty site, and it will be difficult to bring them back if they get a bad first impression (empty site, non-existent community).
Some tactics you can explore:
1. Tap into your network and request participation;
2. Recruit "beta users" who get special perks for being early adopters and contributors;
2. Try crowdsourcing sites for lighter content contributions and community participation*;
3. Hire "sales" people to solicit content for you (this could be anything from offering free initial listings if you are building a directory or identifying appropriate content and asking permission to syndicate on your site);
4. Hire writers to contribute core content;
5. Advertise strategically to drive traffic for potentially more contributors and participators.
People want to write and contribute where they see others writing and contributing.
*While I don't endorse it, some people use sites like Fivver where for a nominal fee, you can get people to do small tasks including contributing content or commenting.