The following content mostly adopted from http://www.googleguide.com/improving_pagerank.html which most of the people doesn’t know about it:
In addition to considering the number of links to your page and the ranking of the linking page, to compute a page’s PageRank, Google considers hundreds of factors including
- how fast a site is gaining links
- how long the links persist
- when your site acquired the links
- the click through rate (CTR) of Google’s search results, cached pages, favorites on the Google Toolbar
- the stickiness of your site (i.e., the effectiveness of your site in retaining individual users)
Most of the websites now are using rel="nofollow" to avoid passing Pagerank weight to the link. However, due to the other factors like above, if still worth you to link it. After all, if you’re getting link from valuable website that can bring in actual visitors, who cares about the pagerank? So stop wasting your time to submit url to link farms.