Over the years I've been either a freelance IT consultant or the head of an IT consultancy. I never got a client from LinkedIn, no one that worked for me ever got a client from LinkedIn, and more broadly, no one I know has ever got a client from LinkedIn. I'm sure they have some purpose (public resume database?) that I'm not clear on, but their value from my perspective is 0.
OTOH, I've gotten several jobs via Twitter. Anecdotal, but Twitter is a more powerful business-oriented social media service if you're looking for a job.