In this piece by Jeff Bullas about why PR should take Social Media Seriously, he compiles some great statistics about the reach of the eight major social media channels through many different sources:

• Facebook: Over 200 million members and 80 billion monthly page views (Techcrunch)
• Twitter: It now has over 44 Million unique visitors a month (according to Comscore)
• YouTube: Over 100 Million visitors in March and over 13.8 Billion video views in March alone (YouTube report)
• LinkedIn: Over 16 million unique visitors a month with a a very affluent demographic where the average user is 39 and makes $139K ayear and over 500,000 C-level members (Quantcast)
• MySpace: Just under 100 million visitors a month and 43 Billion page views (Techcrunch)
• Digg: More traffic than the New York Times with 23 million unique visitors a month and 4.5 billion pages views (Techcrunch)
• Blogs: Over 5 million blogs are tracked by Technorati, the “Bible” of Blogs with 600,000 being corporate blogs (Technorati)
• Wikipedia: Nearly 63 Million Unique visitors June 2009 (Compete)

Then he compares it to the statistics for reach utilizing "old media:"

• “USA Today” (largest read newspaper in the USA)  2.1 Million a day (Audit Bureau of Circulations) which if it was unique views per month of 63.1 million (which it isn’t it.. would still be 3 x smaller than Facebook)
• AARP is the most widely circulated magazine in the USA and reaches 24.5 million which is slightly more than Digg (Audit Bureau of Circulations) but still 8 times smaller than Facebook
• Fox News Channel (highest rated news channel for 90 months) has 15.6 million viewers a day (Huffington Post)..The same as the number of unique LinkedIn vistors sees monthly.
• Rush Limbaugh the largest talk radio in the States  has 14.75 million a day is less than half the 40 million unique visitors to Twitter each month (Talkers Magazine)

It's a pretty stark comparison. Think about that potential audience and then ask yourself, why isn't my business or organization playing in this game yet?