You literally type into Google "I have this problem" and get a bunch of ads solving that problem. It is practically the holy grail of advertising. Now consider that it is a catch all for personal and corporate questions, it is a cash cow. It is perfectly acceptable to google something at your job, click on a link, and buy a $15,000 system.
Facebook might know a lot about you, but they don't know what you are looking for at any given moment. Add on top that most people are spending their time on personal relationships, and it isn't the same even if they get more impressions. I can't log into Facebook from work and say "who has 5" diameter pipe?" and expect a response, but if I google that I find a vendor and place a multi thousand dollar order.
Also the market on Facebook is somewhat inverted. The "best" customers are generally on their the least. The most frequent people are college students - mostly broke at that. If you aren't advertising clothes, personal electronics, or local bars I don't see the draw.