Here are a couple of social networking sites that I've found extremely helpful in building my businesses.
One is
CafeMom and the other is
Natural Networkers CafeMom has hundreds of forums you can join and you can post journals, link vis RSS to your blogs, put up your websites, etc.
Natural Networkers is more of a "how to" on niche business building with many great discussion areas and lots of people who are really helpful.