Why exactly are you looking for PR 5 and 6? I had a short talk with Uncle Google about this just now. Apparently, Pinterest is ranked very highly with PR 9. I have looked at the sites with PR 5 and 6 and I couldn't recognize any of them. So if you are thinking of using a site to create backlinks to your site, I would recommend using Pinterest. It's not difficult to post to Pinterest. Just post some images with links to your site.
These days I am using Stumbleupon the most. It may not be PR 5 level but it can give you some good traffic. And many people are using it for the traffic reasons. Lot of genuine traffic can be found on this network. Apart from that there is also flipboard. That social network has managed to make many site get some regular and decent traffic.