No I really dont think it will improve your PR. Other than the highly needed traffic one should really focus on.
Yes they will and they also bring more traffic too. Here is a good list: http://www.ebizmba.com/articles/social30.html
social bookmarking sites are usually used to increase your site's traffic. but sometimes you may get a stumbleupon profile with good PR linked to your page and that may help you improve your PR as Stumbleupon profiles are counted in google's cache.