That is assuming that the target audience of the sefer is on social media and has internet access.
I think people still find their sefarim by browsing the sefarim store or when being sold in back of the beis medrash/shul.
Maybe it will change, but so far, Amazon is, if at all, a supplement to the traditional means of distributing/selling sefarim, and has not replaced it.
Perhaps the model will change, as more and more frustrated authors really want their work out there, but can’t afford the traditional way.