A useful snapshot -- read the full article
First, despite all the criticism of its privacy policies, Facebook is fundamentally based on the notion of privacy. You cannot find out much about someone unless they have willingly elected to be your "friend," or if they are in a partially-open network you also belong to - for your town or workplace or school. The other reason is that Facebook is intended to be a communications medium. Think of it as, in part, a way to broadcast information about yourself.
Linkedin, by contrast, is a sort of high-end consensual database of colleagues. In some ways it aims to turn the entire planet's workforce into one big set of colleagues, who only come to know one another when one can solve a problem for the other. You can look for that job or find that consultant or employee, because Linkedin's member data is essentially open for all to see, and because the site offers search tools to help you slice and dice it. (They are much more sophisticated and useful if you're a paying member.)