I started an "open blog", everyone can register and post. I give many features like ... adsense in posts, social icons for users , fast aproval on high optimised wordpress blog. Another thing is that users can post excerpt from his blog/website and the link/baner with "read more on my website" if you want to try it visit the link from my signature. Any tips on how to promote it ?
Maybe approach fellow bloggers and tell them about it, either in a comment, or a message, what have you. Obviously, you don't want to spam them, but this is who would be attracted to this website.
One of the first things you need to make sure you do (if you havent already) is seed the site. Plain and simple, people aren't going to be the first to post. No one likes being the first at something..its much easier to get people to post once they see that there is stuff there. So get your business contacts, and friends to go on and post their own stuff first. As for promotions, article marketing would do the trick. I'm not a big fan of it but it does work. Also I highly suggest facebook and twitter marketing. I've always had success their with my interactive sites. Best of luck