The average Facebook user sees only 20 percent of the 1,500 stories per day that could have shown up in their news feed.  The posts you receive are determined by algorithms whose bottom line is Facebook’s bottom line.  The company is constantly adjusting all kinds of dials, quietly looking for the optimal mix to make us spend more of our time and money on Facebook.  Of course the more we’re on Facebook, the more information they have about us to fine-tune their formulas for picking ads to show us.