SEO Book says
Did you know that search.twitter.com/search?q=%23superbowl currently ranks #9 in Google for superbowl (it was #7 earlier today). Think how much PageRank & link equity is needed to rank for that keyword!!!
All that PageRank must come from somewhere. When people mention you on that silly network, you probably don’t get anything of lasting value…it simply steals links that would have occurred on the real web, and replaces them with junk rel=nofollow links, surrounded by trivial bits of content