Long-time Facebook users may remember the good 'ole days, when whatever you posted could be seen by whomever was following you. No matter what you shared, if someone checked their news feed, they'd have an opportunity to notice your post.
Those days are long gone.
Thanks to Facebook's EdgeRank, now, every time you share an update, an algorithm is applied that determines, out of all your current followers, which ones will actually have an opportunity to see it. The algorithm is supposed to be designed to help Facebook users see the posts that are most likely to interest them.