A small clarification:
The reason Facebook has any advertising income, and therefore value as a company, is that the people purchasing FB advertisements believe it has the ability to provide very directed advertising.
Actually, online advertisers know exactly how effective the directed advertising is. Unlike traditional advertising, where the old saw goes "I know 50% of my advertising budget is working, I just don't know which 50%", online ads can very often be linked precisely to specific sales. This is actually the biggest factor in Google's success as an advertiser; good targeting was important, too, but the real breakthrough was being able to help advertisers quantify very precisely what return they were getting on their advertising spend.