How Social sites or some common sites gets more Page Rank and traffic without any link Building or any Promotion Like facebook, Youtube and twitter.

It is a simple matter of "Word of Mouth". In webmaster and seo circles, when a new site comes out that offers a means of free link building people like to share it with their peers. Whether it be talking about it in a forum or blog, word quickly gets around and with this comes a large amount of linking going into the site; hence the large amount of backlinks, traffic and then pagerank.