Phil Rosenberg, President, reCareered posted a good comparison for the strengths and weaknesses of LinkedIn and Facebook - these questions come up so see
his post on College Recruiter - as usual, applies up and down the ladder of experience...
My personal take is I like both, but tend to spend more time with LinkedIn probably because much longer invested time and contacts there. LinkedIn offers daily opportunities to connect people, but both are equally contributing to my companies sales.