It’s a misconception that websites have page rank. Only individual pages on the website have page rank. If someone says they have a PR 4 site, that means that the home page on the site has a page rank of 4, but it doesn’t follow that EVERY page on the site has a page rank of 4. Some pages on the site may have no page rank at all, while other pages on the site could have an even higher page rank than the home page does.
If you are trying to boost your own site by leaving a link back to your website by posting a comment on a blog or website, it certainly would not hurt to do a little checking and determine the page rank of the individual page you are going to comment on. An easy way to find page rank is to use the SEO for Firefox browser extension, which is a free application. Another thing to be aware of is the number of outgoing links on that page. If you find a page that has high PR, but already has a lot of comments on it, hunt around for another page with less comments. The outgoing link juice from the page is divided by the number of links on it. So a page with a bit lower PR, but fewer comments can actually give you more bang for your SEO buck.
Many people strive to only leave comments on “do-follow” sites. This is a mistake as well. It looks far more natural to the search engines if you have a nice mix of “no-follow” and “do-follow” links back to your site.
