[rchaud]

That's correct. If Google treated every Twitter/FB/Reddit link as useless just because those sites attach 'nofollow' to every external link, it would have a much worse idea of what links are relevant and what's not.

Nofollow is still useful for site owners that are using it as intended, and not as a site-wide policy. It's just that most sites do not get a fraction of the traffic a big tweet or Reddit post would get.