PostRank was undoubtedly the best tool for see the social engagement of feed item in a RSS Feed and evaluate the popularity of a link. The problem is no other tool has grew up to be close to PostRank (Previously "AideRSS").
That is why PostRank has become one of the most used plugin for Google Reader to isolate important stuffs from the noise.
That's why couple of years ago, Google bought them and started to research on their ability to integrate it in their various tools. The major tool that Google thought was not Google Reader, rather it was Google Analytics. Google has integrated PostRank in their Social Metrics Tracking at http://analytics.blogspot.com/2012/03/capturing-value-of-social-media-using.html ;
Google has recently sent this email:
It is bittersweet news that we remind our users that PostRank will be sunset on May 1, 2012.
The sweet part is that Google Analytics now allows you to conduct much of the same measurement and analytics that you’ve enjoyed through PostRank. We invite you to access the new Social reports within Google Analytics. If you don't already have a Google Analytics account it's free and easy to sign up. Learn more about the new Social reports in our blog post.
As of May 1st, PostRank.com, PostRank Analytics, Top Posts widgets, and Google Reader plugin, will be taken offline.
The PostRank team
This means that you will not see PostRank as Google Reader plugin any more :-( This means that there will not be any tool in the world that can effectively identify best post from a RSS Feed. Though, I have a question still live in quora about it at http://www.quora.com/What-are-the-Alternatives-for-PostRank-to-Find-the-Importance-of-a-Post-in-a-RSS-Feed ;
But, this also calls for a new product development. RIP, PostRank.