I doesn't matter. If any site has PR 9 then it's sub-pages will not have same PR. It will be counted as common backlink. In my experience only Ezine Artile has PR 6 and many of it's pages have same too. So, many people likes Ezine to post articles.
Google keeps his eye on Twitter cause they know how profitable it would be to place adds on every profile.
My Twitter is PR2... I think having a PR9 page, unless you're Ashton Kutcher or CNN, is almost impossible.
I have a PR3... http://twitter.com/mp2kmag I'm pretty happy about it. The rest of ours are gray http://twitter.com/ericwfrost http://twitter.com/MovingQuotes http://twitter.com/WINDYrealtor Eric