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Linking out is now considered as 'crucial' in fact you should 'link out of' each and every page.
There is no reason to link out from every page and it isn't crucial to rankings. Don't force it. Only link out when it makes sense for your website, your content, and it won't harm your conversion rate.
Back to the original question:
Is having social bookmarking below articles bad?
Does that affect your PR and ranking because you link out?
I've seen no evidence of it, but if you're that concerned you can mask the outbound links via JS, add a rel=nofollow, or employ other methods.