Why Google Should Be Worried About Facebook

Sure Facebook could serve contextual ads on other sites/service based on what they know about their users, except it would violate their privacy policy.

"We do not share any of your information with advertisers (unless, of course, you give us permission)."

Correct me if I'm wrong, but it would be technically possible for a page to read and report back to the originating server what ad appeared on a page (integrated with js or iframe, etc.). This would violate that clause of the privacy policy.