Facebook does not have a captive audience or userbase. Most are young with limited spending power, not to mention very fickle who will likely move on to the next big thing. Facebook hasn't proved a platform for serious advertisers and marketers because Facebook users are not looking to spend, as many users of search engines are. People go on Facebook to mess around with friends and upload photos and what have you.
They may have 900 million users (supposedly) now, but do not be surprised if that figure is a couple hundred million in 3/4 years and declining.