Earlier quoted context omitted.
My company (like many many others) have tried advertising on Facebook, and other social networks (Twitter included). You pay a bunch of money to them, and get almost nothing in return. It's not nearly as effective dollar-for-dollar as using, for example, google advertising... or just investing in using Email Marketing and building a list. (I don't see how any social networks will be profitable long term).
I think specific social networks will be profitable. Such as a doctors social network, or golfers network. I think a larger catch all network has issues as we're all talking about here.
They visit social networks with the intention to be social. Reading an advert, clicking on it, and buying some product is the last thing on their mind.