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I've never actually experimented with different placement, other than different positions on the bottom of posts. I can say that with one fairly popular site I run (25k visits/day) Faceook, Twitter, and email comprise just about all of the share clicks. Of those, Facebook is clicked about twice as often as Twitter, and Twitter a bit more than twice as often as email. Despite that giant Facebook sharing lead, Twitter sends just about the same number of incoming referrals as Facebook.