Although (and I've said this a buncha times) it does not pass link juice there are many ways that nofollow links contribute to SEO value and in the long run do end up resulting in passing link juice by visitors re-sharing content straight from the site to a blog, social bookmarking site, etc.. Also analyzing it on a computational end and where the search engines are heading with their updates, we are beginning to use data aggregators that the government uses (ikanow offers open source software that big companies pay 100k for, wink wink) to gather info on how Google's spiders are operating.
The fact is nofollow and dofollow were most contributing with the PageRank algo. I've noticed with death of the PR that social shares are having much more SEO value nowadays when mixed with a solid dofollow linking strategy. All of my niches are products in the "cool stuff" category, so social media has been one of my biggest strategies. It was better 4 years ago, but we haven't changed a thing and I am noticing our rankings climbing again.
The problems occur when novices hear that a social share is nofollow. So they get a ton of dofollow links, outnumbering their nofollows and boom, a fooprint. It varies form niche to niche.
Lets's use an easy example:
Site A: Funny meme site, tons of social engagement, but minimal dofollows. <<<< This will rank well.
Site B: Funny meme site, little social engagement, but a lot of dofollows <<<< obvious footprint
One thing to take into consideration is that Googles algo adjusts to human behavior and generalizes. So with all the info out on the web about dofollows being the way to go, you can guarantee novice marketers are going crazy buying backlink gigs that broadcast "dofollow" links. High quality or not, Google's behavior in the past indicates that we maybe heading for an update that slaps sites with too many low quality dofollows, whether natural or not.
Look at 6 months ago for instance when Press Release sites got slapped and lost over 60% visibility in the search engines. That was traumatic. But we saw it coming, because every blackhatter in the world was using press releases for ranking. It was only a matter of time before Google's algo adjusted to the "trend" of blackhatters and using PR sites for ranking.
SEO is a dodgy game and is changing all the time with blackhatter behavior.
That's the secret. Want to learn what not to do? Listen to what 90% of marketers are saying when they talk about "gaming" the system. i.e. link building.
We have clarified which type of backlink you are asking about, "nofollow". 
