Facebook is a social platform trying to learn how to be a business and have their folks do business. They WILL succeed in time.
LinkedIn is strictly business. Socially driven but all business and getting more and more important.
They are not the same and should not be used the same or even seen the same. But both, along with Twitter, have a place in the eco-system of Internet business.
I stumbled on this at LinkedIn and it is beyond valuable. I have talked about it many times over the years. Here is another perspective.