The difference is that Google's various products, unlike Facebook's, provide data that isn't strictly necessary for their ads to be relevant. As their giant pile of money attests, simple targeted advertising based upon search queries is more than enough to build a giant empire upon.

They won't wither on the vine if their various other session-based tracking datasets, such as those you mention, are cut off for whatever reason. The same can't be said of Facebook's trove of private information they're apparently banking on selling to the highest bidder.

The bottom line is even if there are some short term successes for Facebook via targeted advertising from social network data, they will live in constant fear of a bad news day causing all Facebookers to log into their account and uncheck the "share my personal information with 3rd party sites." It could be an overnight death, at any moment.

Those folks might also kill their GMail accounts for the same reason, but they'll still be anonymously searching Google for information and getting relevant ads.