A short time ago (Well actually several months ago), digg comments used to be follow. That is, if you posted a link in a comment for any story, google would pass on a bit of PR juice. That was until “The Digg Comment Party” ended. Do note, this name was only given posthumously after it was all over and the shit had hit the fan at Digg offices. Before then, very old submissions on digg (Ones a year old or so), would carry around up to a 7 in PageRank. Once you found these story’s, you could leave a comment on it, plant your link. And there you had a free high PR backlink!
Just to show you what i mean, go to the digg front page. Select a category from the top menu.
In the date range, select 365 days. This will show all posts that were done in the past year.
Now the submissions shown are ranked in the most diggs to the least. So just scroll to the bottom of the page, and select the last page. For me its 547, but just select the last one 
Starting at the bottom of the page working up. Open up each of the comments pages up and check the PR.
To be honest, Nowadays any crap gets sent to digg so you will be lucky if you find a PR 7 now. But you can certainly find a few 5′s and definitely a few 4′s. Remember though that the PR is worthless as all links left in the comment box are no follow. But anyway back to the story. It lasted quite a while, with countless ebooks over at DigitalPoint being sold with it, and even a few on warrior forum.
The spam got a bit heavy on digg, and you couldnt find a story that hadnt already been hit with plenty of comments pertaining to viagra links. I believe the same thing was once possible with flickr aswell. Finding a photo page with high pr (Easy), and then leaving a comment. Images were still able to be used in flickr comments so they were a handy way to disuise your links. There is definitely social networks out there that you can still find high PR pages that you can comment to, you just need to find them.
