As has already been explained to you, you cannot have a site with high PR; one page may have high PR, and other pages very low PR. In particular, any page where you are able to place a link for yourself is likely to have low PR. And, of course, if you try to do this on any kind of scale, you run the risk of violating Google’s guidelines on link schemes.
I’m afraid I don’t understand your thinking here.
[quote=“IT-INDIA.com, post:1, topic:214632”]
The backlink must be Do Follow unless the PageRank is above 5.
[/quote]If the link is nofollow, it will make no difference to you whether the page it it on has PR0 or PR9; it will be ignored by Google for purposes of ranking your site.
The main purpose of social bookmarking is to allow people to share useful content. If your content is good, then sharing it on social sites (or, better still, other people sharing it on social sites) may well bring you increased traffic. It will not help your SERPs ranking.
I’m hesitant to allow this topic to remain public, as experience has taught us these sorts of questions usually attract responses of long lists of sites, and simply encourage Spamming of those site. (Such lists are generally also very out of date. SitePoint regularly appears on lists of forum supposedly providing followed links, yet external links have been nofollow since before I joined five years ago.) The moderators will therefore be keeping a close eye on the direction this topic takes.