Years ago I met with a business owner looking for a web designer. Durham Ontario tends to be slower to embrace new ideas than say Toronto. So I was surprised and happy to hear that this owner was excited about Facebook and it's business applications. But then he said he believed that Facebook was going to replace all websites and the need for them. I disagreed, maybe a bit too strongly, and subsequently never did get the project.
Why did I so adamantly disagree? For one core reason: businesses like control. Why would any business reliquish control of their Internet window to the world to a third party? Although Facebook is used by many, it's a closed platform and makes it’s own rules. Facebook changes when it wants. (Anyone have to redo their business Facebook page after Timeline?)
Facebook has it's place in the business world, but it's a marketing tool not a replacement for your website.