The Enthusiast Group is an exciting company built around providing social networking and citizen media platforms for passionate folks. For nearly 2 years now they've been building sites to support communities around adrenaline sports (YourMtb.com, YourClimbing.com, YourHorseSports.com, YourCycling.com, YourRunning.com). This has given them great experience in the world of social networks and user-generated-content (citizen journalism) including some relatively novel uses like Grassroots coverage of major events.
They just released a white paper (email submission required, but I'll vouch for them not being spammers) and even though my job and hobby is working with social networks I still learned a lot from the document. It's a 31 page pdf that provides some great resources: background on the topic including definitions of some key terms and phrases, real-life case studies of a variety of sites, and links to other valuable resources on the topic.
At the same time they've also started offering a new set of services that includes building a specific social network/grassroots media site for any brand, cause, or focus. It's a really neat idea and exciting to see them growing like this. Much like "books were just the beginning" for Amazon, it appears that adrenaline sports were just the beginning for Enthusiast Group.
Oh yeah, and of course it's all built on the best php cms.