What they said, although I set my facebook feed to “show me everything” as i thought it unfair that I didn’t get to see everything that was being posted by family/friends.
It’s crap, the filter works.
As others said, Facebook infers whether people actually want to see your posts or not based on clicks, likes, comments, shares etc. So if all you did was post “Merry Christmass”, and 600 people saw it, no one replied and no one shared it (why would they?) then Facebook probably deemed it to be boring spam.
Which is annoying because some shops are really good at playing the system, asking inane questions in every post, begging for “likes and shares” by offering a vague promise of future special offers or competitions. So you tend to get the same crap day after day until you un-follow them. Whereas the ones that actually post some interesting news/blog/whatever get buried.