So, Google acquired PostRank, the service calculating impact of blog posts and other items in social media.

If you want something similar but without the Google tie-in, then a good option is my social impact calculator which is fully free software written in PHP. It was originally written in 2007, but the newer version has been cleaned of Midgard dependencies and updated to reflect the current popular social networking services. Usage example from my earlier post:

require('calculate.php');

$url = 'http://bergie.iki.fi/blog/introducing_the_midgard_create_user_interface/';

// Get the raw count for only one source
echo com_meego_planet_calculate::hackernews($url); // 145
echo com_meego_planet_calculate::facebook($url); // 1

// Get weighted total score for all sources
echo com_meego_planet_calculate::all($url); // 130.8