Linked in is definitely for professional networking, although the extent to which you can succeed with that I'm not sure - although I think that mainly reflects my own lack of understanding of how to get the most out of it.
Facebook is, in my view, purely for my personal social networking. Having said that, I have ended up with a couple of clients as 'friends', so I'm always mindful that I am networking with them to a degree. I'd prefer to keep the two separate really, but when I made a Facebook page for my company I inadvertently linked it to my personal page, hence how they came to be 'my' friends!