May be it has to do with the fact that number of employees in google are many times more than that of Facebook, bringing down the compensation avg as well as the avg job satisfaction.
The pre-IPO part is key. They still have a ton of employees motivated by the "I'm about to be rich" carrot. Facebook is young and growing, it's easy to have satisfied employees at this point.
It's keeping them that way over the long haul that's the trick (particularly after the 'sexiness' has worn off).