Actually, it is essential for Facebook to have its content crawl-able by everyone - which is why there is the constant push towards downplaying privacy. A quick look at Facebook on both Compete and Alexa says that they get ~8% of their referral traffic from Google alone (the search engine is also their largest referral traffic source) and the Google properties together contribute referral traffic in double digit percentages.

"Social" itself is not a product. It is almost like gasoline - by itself either product does not give you any value, but use it as a fuel or an enabler and it takes on a different form. Facebook's enablement is primarily through connections (the gateway drug) and later it transforms itself into a tool to easily eat up plenty of your time at varying levels of interaction/engagement. As much as Google wants to tap into these primary and secondary objectives now owned by Facebook, Facebook itself would want to move beyond these two.

Facebook has to eventually become the arbiter of transactions online, which has to be a full-on solution than the teetotaler approach taken on FB credits. That would, though, expose Facebook to higher standards on privacy. But, if they manage that, Google can be easily overrun.

For Google, "social" cannot be force-fed. "+1" is such an eyesore and is such a horrible effort at branding your social interaction touchpoint for the masses.