Recently Mitch Joel at TwistImage talked about why the content created by many companies is not being seen on Facebook, Twitter and other social networks:

http://www.twistimage.com/blog/archives/almost-no-one-is-seeing-your-content/

In this episode I expanded upon this a bit to note that you have to remember that these companies are NOT the "open Internet" and are providing services based on their business plans... which are open to change whenever they want to do so.