While I was reviewing and clearing out what I had tagged to Del.icio.us over the year, I ran across this post from Robin Good on his blog. Robin has really created a comprehensive overview of how to measure and ensure Popularity, Authority, and Credibility online.
Check out his PACmeter blog from 11 August 2004. There are links to several academic papers and social network media tools.
Robin's blog is always rich with relevant content dealing with knowledge management, communication and the art of doing business in the world today.