Google has a social network and a [mobile] operating system.
  Who’s going to have the best mobile social user experience?

While this is an attractive bet, as Google you would have to hope that neither your OS or network got too popular or you could quickly end up in anti-trust territory. Android being open-source changes things slightly but it doesn't solve your competitors' problem (deep integration access to your own stock devices).

  Zynga’s IPO filing shows $597.5M in revenue and $90.6M in earnings 
  in 2010. If Facebook had around $2B in revenue and $250 million in 
  earnings in 2010, and 99% of Zynga’s revenue comes from Facebook, 
  the math says Zynga could be nearly a third of of FB’s top and 
  bottom line.

That's some funny accounting.

  $1 invested in Google (at a $167B market cap) will make it to $2 
  before $1 invested in Facebook (at $80B market cap) makes it to $2.

That's not so funny.