You can say LinkedIn is social networking for professional. Though it don't offer always free service. It has paid service also.
LinkedIn is a great resource and it covers all sorts of industries. There is a lot more to it than meets the eye and you can actually take part in conversations with people in your industry and target market. I wrote a blog post aimed at small businesses with some tips on how to get the most out of it so it might be worth a read: http://thedigitalpost.co.uk/2011/03/29/linkedin-for-small-business-are-you-taking-advantage/