There are many interesting discussions happening on Twitter in the last few weeks. I have been paying attention to the conversations about how to measure social media influence/rank/authority. Many people trying many metrics. All of the current measures seem to have their issues... one gives too many high scores, another spits out way too many around the median, and still others are easy "to game."
IMHO, the best metric so far is Retweet Rank -- it is not perfect, nor complete, but it gets at a key aspect of Twitter -- what is interesting, what are people paying attention to, what is useful and who is sharing all of that. If someone re-tweets what you posted/tweeted then that is a vote of attention/quality -- if many people re-tweet you, or the re-tweet the person that already re-tweeted you, then better yet! In this sense Retweet Rank follows the example of Google's very successful PageRank algorithm -- people point to web content they find interesting/useful/valuable.
Pistachio talks bout what T has meant to her