Google is a search site and purports to have close to every publicly available(i.e not banned in robots.txt) page indexed. Delisting a very popular site(will be doing that for the first time) will get it into the news for the wrong reasons. And how much will it hurt Facebook? Barely. How many people signup for Facebook that searched "Best social networking site" ? It will actually hurt them monetarily so it will be hard to sell to stockholders. I see almost zero chance of this happening.
On the other hand, Facebook's value is not in showing ads, it's about social networking. How will removing Google ads hurt it? Will it kill off enough FB applications for people to take notice and leave FB for... what? Orkut? Google Go? Myspace?
After all, the FB applications use Facebook's resources, so they want to indirectly charge them for it while making money.
Indexed is not the same thing as available, or else Google would not be able to tweak their algorithm against so-called spam networks like Demand Media.
Historically, Yahoo trumped itself as having the best search results thanks to human curation of its search directory. Bing, even today, labels itself as a "decision engine".
Google as far as I know has no public statements implying that there's a 1:1 correspondence between the set of possible Google results for all queries and the internet. This relationship is far more important than merely a 1:1 between index and internet.
