A very interesting article really. The niche point that you have highlighted is a good one. A new website or even a new article, if targeted to people across the globe is surely going to get lost and go down the drain whereas if the same website is targeted at a particlular niche segment, then it's obvious that people are going to share it and talk about it in their circles.
This contributes in a major way to your link building campaigns as well the reach of your business increases and slowly you can build more and more on that.
But with various social media sites popping up, will Google consider this social form of link building also while ranking websites. It surely will I guess but only when it completely overhauls its system and puts some effective tracking analytics in place. But there will surely come a time when social media will step up and will hog the limelight from SEO.
Here's an interesting article I would like to share with fellow social media today users - http://bit.ly/zE2L4t