Ning.com is a platform that lets you create your own social network for anything. It has been experiencing phenomenal growth and just recently passed the 170K mark.
Manny and I became acquainted shortly after he created, TuDiabetes.com, a social network for people touched by diabetes ( I have Type 1 diabetes ). TuDiabetes is among hundred of community sites created with the Ning.com network system. Ning allows anyone to create a social network site for free within minutes of creating an account. It features integration with Flickr, Facebook and YouTube for members to personalize their experiences with the community. Community builders or administrators have access to powerful SEO and other analytics tools for building out and branding their network.