aristotle - 4:42 pm on Jun 20, 2013 (gmt 0)
This topic has been discussed many times. Many big sites, especially social sites, automatically put nofollow tags on all external outlinks on the assumption that it discourages link spamming, or at least prevents link spammers from getting any SEO benefits from their activities.
one question that is sometimes discussed is whether Google's algorithm includes nofollow backlinks as part of its evaluation of the naturalness of a sites overall backlink profile. For example, if a site has a thousand dofollow backlinks but only 1 nofollow backlink, it wouldn't look natural and therefore could imply that something is amiss, so that the site isn't as trustworthy as it otherwise might appear to be.