I don't know honestly, it should be that way but isn's always. There are websites I visit often that have crap content, copied from other websites or Wiki or whatever and they have higher google rankings or whatnot then a great site with original admin/staff written content. I think it depends on the user groups and many other factors
I do understand you point but we should not neglect the importance of content. Even Google recommend to have good content on your website to attract more organic links. It is our duty to promote our quality content over social network sites and get attention from other users. If the content is good we will get links from them or else we lost.
Content is a factor which matters more for Search engine spiders but not for an automatic traffic. But yes if you are writing some interesting stuff then yes people will do some viral marketing of your written material.