Why do contextual ads fail

I crowdsourced some questions about contextual advertising and contextual content on Google+. It was an unscientific survey, of course. But several strong consensuses formed that perfectly matched my own observations.

The strongest consensus was that Facebook advertising is off target and almost completely irrelevant.

The question is: Why Facebook has a database of our explicitly stated interests, which many users fill out voluntarily. Facebook sees what we post about. It knows who we interact with. It counts our likes, monitors our comments and even follows us around the Web. Yet, while the degree of personal data collection is extreme, the advertising seems totally random.

What is Facebook doing with all that personal data

When I go to Facebook, I never see an ad that demonstrates the company's intimate knowledge of me and what products I might want to buy.