This discussion confirms my assumption on link building.
I have never worked (never will) on link building for my sites. The maximum is the little social buttons at the bottom of articles.
I haven't even looked for the sites linking to me.
The user engagement, branding, social awareness plus user experience are supposed to work but still I am not sure how/if these metrics affect ranking and in what extent.
I think SE (especially Google) are still struggling with the "social data" they gather from the web. Messi Panda is the result of their efforts.