I had lunch today with Justin Perkins, Super Nice Sales Guy and Consultant for Care2. Care2 is a for-profit, niche, social network that helps over 8,000,000 people who "Care2 make a difference - Discover, Share, and Take Action!"
What is a niche social network you ask? Think of it as semi-private party vs. a house party. Unlike a house party (i.e. MySpace), Care2 and other niche social networking sites cater to a specific demographic or issue. By focusing on a specific topic or personality-type, niche social networks become more attractive to marketers who need to reach a specific audience, i.e. women with young children, environmentalists or dog-lovers. (BTW, this IS the demographic on Care2 aka LOHAS - Lifestyles of Health and Sustainability.)
Based on Care2's profile, here are three types of nonprofits that might want to join their network.
1. Progressive organizations might introduce their advocacy platforms to Care2 members.
2. Womens' organizations might invite Care2 members to join their Giving Circles.
3. Youth development groups might enlist Care2 volunteers.
You get the point.
Before wading into the social networking space, do some research to find the niche social network for your target audience; this way you're likely to reap better marketing results.
Here is a list of social networks on Wikipedia.