Seeing as Youtube and Digg have a PR of 8, wouldn't you're site get a big increase in rank if you had links to your websites from these sites? Why would you pay for 1 permanent link to a PR8 site when you could just post it on Digg, or Youtube???? Doesn't make sense. And even big forums, like this one. Isn't having your site in the signature exactly the same?
Generally most of these sites adopt a ref=nofollow attribute hence why they have such a high PR. Also, if they do then this will not benefit your SEO on your site as the PR will not get passed down to your site.....
But the benefits from getting links from these websites is traffic. You might not have the PR juice but traffic might result in place of that.
Oh ok thanks for the info. I have another question. Does it make a difference in the amount of links that appear on 1 site? For example is having 10,000 posts with your site in your signature in this forum better than having 1 post in this forum? In otherwords is it true that the more links you have on 1 website the higher your PR will be also?
The deeper the page, the weaker the link. Even if you have a link from these to your website, it will be a buried link which search engines will count as a weak one. I'd advise you to have your links by 0-2 clicks from the homepage. My 2 cents anyway
Those sites are all about traffic, not PR. You're link wont stay on the front page long enough to get any PR juice. cheers