As a simple reality, FB has been reducing page real estate allocated for organic things.  This data below via @dennisyu shows the pivot point where paid inventory gradually replaced organic on FB templates.
The greatly reduced organic visibility mostly limits friends-of-friends (FoF) amplification, the phenomena which helped fuel FB’s meteoric rise, has reduced FB’s brand page to a mostly closed loop system. In other word, when a brand’s friend interacts, THEIR friends don’t see much of the interaction.
When a brand posts, only a percentage of the brand’s friends (1rst degree) actually see the initial post. That’s the bad news. The good news is that organic prominence can be bolstered and even smashed, with a little cash for sponsored stories and FB Ads, along with an an realizing how FB really works (the modern incarnation of an email list). This understanding of the FB distribution network means that marketers can clean up in FB…by the venerable subscription model. Read on for the details.
Anatomy Of  A FB Content Release
