Digg, diggo, reddit, stumbleupon, delicious, folked, simpy all are high pr and dofollow social bookmarking sites, for more list you can also search in google..
Twitter,Digg and Facebook is mostly preffered for bookmarking high PR sites.
Twitter and Facebook are a waste of time unless your link proves very popular and gets passed on/retweeted. Otherwise search engines treat them as almost meaningless.