social media's role in link building?

I have been lurking around here for a few months and have picked up a lot so far... as the title suggests, I still don't fully understand what social media's role is in link building. I of course get the idea of driving traffic direct from a social media form to a money site. But... shares, posts etc... do these help in indexing new links? Do they count significantly towards link juice? Do they help in indexing links? I would be very interested to hear your input about this.