A Green Road Blog 

http://www.greenbiz.com/column/2008/07/25/company-benefit-green-social-networking

Here is a list of the top 5, by Alexa Traffic Rank, accompanied by each network's Google PageRank.

Graph available on link above

"There are a number of ways marketers can integrate social networks into their marketing mix:

• Create branded profiles and start groups on existing social networks. Users often connect to a brand or a cause as an expression of their online identities.
• Integrate marketing campaigns with social network profiles and tools. Extending your marketing efforts to online social environments can help lead to viral distribution of your marketing message.
• Associate with celebrities. A celebrity link to your branded page equals a tacit endorsement -- but be wary of backlash from skeptical consumers. This relationship tends to work best when the celebrity's environmental credentials are well established.
• Start your own social network. Online applications such as Ning have greatly simplified the process for creating your own network, including features such as blogs, video, and interactive tools or "widgets."
• Advertise on social networks. Social networks are looking to generate revenue by offering more targeted opportunities for advertising. Facebook, for example, now enables marketers to advertise based on users' profiles."