Why isn't Oprah on Facebook?

Here is an article that is all about brands that embrace social media, quietly.  PBS is listed as starting social media because of the engaging and educational programming like "This Old House".  HGTV, Oprah and ESPN round out the rest of the list.

Oddly enough, some of these influencers are more quiet than others.  It's odd to me that Oprah is not on Facebook.  There are several fake Oprah or Harpo facebook pages, but nothing dedicated to the official, original influencer herself.  Furthermore, a preliminary Twitter search produced only an "O" home magazine tweet.  O home magazine can maybe live on online, since the hard copy is now defunct.  

Myth Busters on Discovery Channel, was not on this list, but is an influencer in my eyes.  The show could not go on without viewer input and participation.  In terms of engaging, conversational and educational criteria, it seems that Myth Busters has (like PBS) been using social media way before it was even called social media.  And although the show is popular (it's on broadcast TV!), Myth Busters is not on Facebook (officially) or on Twitter-two of social media's largest networks.

Why aren't the real social media influencers or inventors like Oprah and Myth Busters on the two biggest social media networks?  It seems like they have nothing to lose, and can only gain influence and popularity. 

Just a curious observation.