Earlier we shared an article why Facebook Share button missing on posts shared to only Friends which is a part of privacy control. In this article, we have shared to you why Facebook share button showing on some posts especially on external links.
Facebook restricted posts that are set to be available for friends from being shared by any other person. When someone shares a post set only for friends, according to Facebook the post will only be visible to friends that you are sharing with the owner of the post you shared.
But Facebook has modified post’s privacy to allow other friends to share posts linking out of Facebook. Since links are for public views, then someone else can also share the same link whether the privacy is set to Public or Friends.
How Do I make Facebook Share button showing on normal posts?
The only way for now until Facebook changes mind in later updates is to change post privacy to Public. Public means that you want your friends of friends, non-friends to see your post even when shared by your friends or their friends of friends.