Social networking sites provide quality backlinks similar to directories. However social networks have the advantage of sending traffic to your website. I heavy traffic from several sites . Many bloggers will submit your site to these social networking sites once others send each and every one can get nore back link.
Personally, I would like to recommend blogging and press releasing for quality backlinking purpose since these 2 channels would not only be helpful in the quantity but also quality of backlinks to your sites. They are both contextual backlinks and would be loved by all search engines.
Blog commenting and forum participation will get you quick backlinks. Making other sites and blogs voluntarily will bring you more quality links. To get that, write quality contents that will compel the other webmasters to link to your blog. The two most important things of SEO is writing good contents and continuously putting your efforts to get back links. This will bring traffic over the period.