People who chase high PR links have no idea what they are doing or talking about. A long time ago PageRank was very important and a high PR link meant high rankings. But that hasn't been true in a long time. Google has improved their algorithm tremendously since then and other more important signals, ones that actually make sense, factor into a link's value.
Some of those factors include:
- The content of the linking page (the more closely it matches the content of the linked to page the better)
- The ranking of the page giving the link for the content of the linked page (related to point #1)
- The anchor text of the link
- The text surrounding the link
- With Panda and Penguin you can infer the quality of the overall website is a factor
There's more but that should give you an idea of what Google is looking at and why chasing PR is a waste of time.
(FYI, PageRank is per page, not per site. So Twitter is a not a PR8 website. It's homepage has a PR of 8).