With all of the free tools available to develop a social network, creating a community has never been easier. Engaging and keeping your community active however is a different story. Mashable posted
le.com/2009/12/16/community-engagement/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed: Mashable (Mashable)&utm_content=Google Reader”>10 Rules for Community Engagement get you started.
You also might want to read The Art of Community by Jono Bacon.
Bacon is the Community Manager for Ubuntu, one of the largest open source software projects on the planet. In this book he talks about the ins and outs of building, cultivating and managing a community from the ground up. This is a must-read for anyone interested in community development. In a truly community-friendly effort, you can download the entire book for free and share and modify it under the Creative Commons Attribution Share-Alike Non-Commercial license. -Mashable