How many people actually care whether their friends like (whatever "like" means) Coke or Pepsi? The most useful graph searches would most likely be job or travel-related queries like "friends who live near San Francisco" or "friends who work for Google". Such truly personal data will never be polluted by advertising campaigns.

And if "friends who like Restaurant X" doesn't turn out to be a reliable way of getting a good restaurant review, people will quickly stop using it. Or they'll send a message to their friend to confirm whether they really recommend the restaurant or they just liked it for a coupon. Eventually they'll figure out which of their friends are reliable reviewers and which just click on every "like" button.