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I'd advise you firstly look at the purpose of the nofollow attribute to answer this one. When you nofollow, you're telling Google that you don't want to pass your page's reputation to the pages linked to. There's therefore no reason why you would want to nofollow the social profiles. You want to give reputation to these pages, you want them to rank for the brand etc. It wouldn't cause "over-optimization" signals but it's pointless nofollow'ing such links.