Two prominent social media marketing executives have gone on record to accuse Facebook of quietly altering one of its key algorithms in September, so that companies with pages that have large numbers of followers can now only reach a fraction of the followers they used to with each post.
http://www.businessinsider.com/facebook-changed-edgerank-algorithm-to-hurt-advertisers-2012-10
and also:
http://www.adweek.com/news/technology/another-agency-claims-facebook-algorithm-changes-144405
