We tried FB ads at a company I've worked for. The return was so bad we canceled the whole campaign. Compare that to Google SEM, which is the largest channel by far.

FB may have a ton of data about you, but no amount of demo data can beat a person who searches for "i want to buy x" when you're selling x.

Interestingly, Google has had the same difficult time monetizing gmail (compared to search), even though they are targeting based on the content of your inbox.